feat: add metrics collector endpoint to libstorage (#1454)

Co-authored-by: Marcin Czenko <marcin.czenko@pm.me>

Makefile

@@ -85,6 +85,7 @@ endif
 	test \
 	testAll \
 	testIntegration \
+	testLibstorageC \
 	testLibstorage \
 	update
 
@@ -152,7 +153,7 @@ testIntegration: | build deps
 		$(ENV_SCRIPT) nim testIntegration $(TEST_PARAMS) $(NIM_PARAMS) build.nims
 
 # Builds a C example that uses the libstorage C library and runs it
-testLibstorage: | build deps
+testLibstorageC: | build deps
 	$(MAKE) $(if $(ncpu),-j$(ncpu),) libstorage
 	cd tests/cbindings && \
 	if [ "$(detected_OS)" = "Windows" ]; then \
@@ -163,6 +164,10 @@ testLibstorage: | build deps
 		LD_LIBRARY_PATH=../../build ./storage; \
 	fi
 
+testLibstorage: | testLibstorageC
+	echo -e $(BUILD_MSG) "build/$@" && \
+		$(ENV_SCRIPT) nim testLibstorage $(TEST_PARAMS) $(NIM_PARAMS) build.nims
+
 # Builds and runs all tests
 testAll: | build deps
 	echo -e $(BUILD_MSG) "build/$@" && \

build.nims

@@ -69,18 +69,17 @@ task storage, "build logos storage binary":
 task mixTools, "build mix tools (mix_pool, mix_relay_dht)":
   let (desc, ec) = gorgeEx("git describe --always --dirty")
   let mixVersion =
-    if ec == 0 and desc.strip().len > 0: desc.strip() else: "unknown"
+    if ec == 0 and desc.strip().len > 0:
+      desc.strip()
+    else:
+      "unknown"
   let mixParams =
-    "-d:chronicles_runtime_filtering -d:chronicles_log_level=TRACE " &
-    "-d:mixVersion:" & mixVersion
+    "-d:chronicles_runtime_filtering -d:chronicles_log_level=TRACE " & "-d:mixVersion:" &
+    mixVersion
   buildBinary "mix_pool",
-    outName = "mix_pool",
-    srcDir = "tools/mix/",
-    params = mixParams
+    outName = "mix_pool", srcDir = "tools/mix/", params = mixParams
   buildBinary "mix_relay_dht",
-    outName = "mix_relay_dht",
-    srcDir = "tools/mix/",
-    params = mixParams
+    outName = "mix_relay_dht", srcDir = "tools/mix/", params = mixParams
 
 task testStorage, "Build & run Logos Storage tests":
   test "testStorage", outName = "testStorage"
@@ -94,6 +93,9 @@ task testIntegration, "Run integration tests":
   # test "testIntegration", params = "-d:chronicles_sinks=textlines[notimestamps,stdout],textlines[dynamic] " &
   #   "-d:chronicles_enabled_topics:integration:TRACE"
 
+task testLibstorage, "Run libstorage Nim tests":
+  test "testLibstorage", outName = "testLibstorage"
+
 task build, "build Logos Storage binary":
   storageTask()
 

library/libstorage.h

@@ -120,6 +120,13 @@ extern "C"
         StorageCallback callback,
         void *userData);
 
+    // Returns node metrics in the Logos openmetrics-compatible
+    // format (https://github.com/logos-co/openmetrics-module).
+    int storage_get_metrics(
+        void *ctx,
+        StorageCallback callback,
+        void *userData);
+
     // Set the log level at run time.
     // `logLevel` can be one of:
     // TRACE, DEBUG, INFO, NOTICE, WARN, ERROR or FATAL

library/libstorage.nim

@@ -185,6 +185,19 @@ proc storage_peer_id(
 
   return callback.okOrError(res, userData)
 
+proc storage_get_metrics(
+    ctx: ptr StorageContext, callback: StorageCallback, userData: pointer
+): cint {.dynlib, exportc.} =
+  initializeLibrary()
+  checkLibstorageParams(ctx, callback, userData)
+
+  let reqContent = NodeInfoRequest.createShared(NodeInfoMsgType.METRICS)
+  let res = storage_context.sendRequestToStorageThread(
+    ctx, RequestType.INFO, reqContent, callback, userData
+  )
+
+  return callback.okOrError(res, userData)
+
 ## Set the log level of the library at runtime.
 ## It uses updateLogLevel which is a synchronous proc and
 ## cannot be used inside an async context because of gcsafe issue.

library/logosmetrics.nim

@@ -0,0 +1,66 @@
+import std/[json, locks, strutils, times, sets]
+
+import pkg/metrics
+
+proc jsonHelp(collector: Collector): string =
+  let prefix = "# HELP " & collector.name & " "
+  if collector.help.startsWith(prefix):
+    return collector.help[prefix.len .. ^1].strip(leading = false, trailing = true)
+
+  return ""
+
+proc jsonType(collector: Collector): string =
+  let parts = collector.typ.splitWhitespace()
+  if parts.len == 4 and parts[0] == "#" and parts[1] == "TYPE" and
+      parts[2] == collector.name:
+    return parts[3]
+
+  return "unknown"
+
+proc toJson(collector: Collector, metrics: var seq[JsonNode]) =
+  # We know the closure won't outlive `metrics` so this is
+  # an acceptable hack.
+  let metricsPtr = addr metrics
+  let help = collector.jsonHelp()
+  let typ = collector.jsonType()
+
+  proc serializeMetric(
+      name: string,
+      value: float64,
+      labels: openArray[string] = [],
+      labelValues: openArray[string] = [],
+      timestamp: Time,
+  ) {.raises: [].} =
+    # The logos openmetrics format (https://github.com/logos-co/openmetrics-module)
+    # does not include the timestamp, so we don't include it either.
+    var labelMap = newJObject()
+    # When a label is missing, it's simply not included in the values, so we take
+    # the minimum.
+    for i in 0 ..< min(labelValues.len, labels.len):
+      labelMap[labels[i]] = %labelValues[i]
+
+    metricsPtr[].add(
+      %*{"name": name, "type": typ, "help": help, "value": value, "labels": labelMap}
+    )
+
+  collector.collect(serializeMetric)
+
+# Serializes all collectors in a given registry to a Logos openmetrics-compatible
+# format. Allows including only specific collectors by name.
+proc toJson*(
+    registry: Registry,
+    exclude: openArray[string] = [],
+    includeOnly: openArray[string] = [],
+): JsonNode =
+  var metrics = newSeq[JsonNode]()
+  withLock registry.lock:
+    for collector in registry.collectors:
+      if exclude.len > 0:
+        if collector.name in exclude:
+          continue
+      if includeOnly.len > 0:
+        if collector.name notin includeOnly:
+          continue
+      collector.toJson(metrics)
+
+  result = %*{"metrics": metrics}

library/storage_thread_requests/requests/node_info_request.nim

@@ -5,6 +5,8 @@ import chronos
 import chronicles
 import confutils
 import codexdht/discv5/spr
+import metrics
+import ../../logosmetrics
 import ../../../storage/conf
 import ../../../storage/rest/json
 import ../../../storage/node
@@ -18,6 +20,8 @@ type NodeInfoMsgType* = enum
   REPO
   SPR
   PEERID
+  # Not sure this belongs here but for now OK.
+  METRICS
 
 type NodeInfoRequest* = object
   operation: NodeInfoMsgType
@@ -74,3 +78,22 @@ proc process*(
       error "Failed to get PEERID.", error = res.error
       return err($res.error)
     return res
+  of METRICS:
+    # We're making a few assumptions here, which need to be double-checked:
+    # 1. That defaultRegistry and all collectors we're using were
+    #    created on the host thread that called libstorageNimMain as globals
+    #    were initialized.
+    # 2. That collectors themselves are thread-safe.
+    # 3. That reading from defaultRegistry from this (worker) thread is safe.
+    #
+    # For (3) in particular, we lean on chronos_httpserver which does the exact
+    # thing we are doing here: reading defaultRegistry and its collectors and
+    # metrics from another thread.
+    #
+    # TODO double-check these assumptions.
+    {.cast(gcsafe).}:
+      # Excludes nim_runtime_info as dumpHeapInstances seems to be returning
+      # an infinite number of objects (could be related to some assumption
+      # failure above).
+      # FIXME figure out what's going on and add this back.
+      return ok($defaultRegistry.toJson(exclude = @["nim_runtime_info"]))

storage.nim

@@ -95,8 +95,7 @@ when isMainModule:
         config.dataDir / config.netPrivKeyFile
 
     privateKey = setupKey(keyPath).valueOr:
-      fatal "Failed to set up the network private key",
-        path = keyPath, err = error.msg
+      fatal "Failed to set up the network private key", path = keyPath, err = error.msg
       quit QuitFailure
 
     server =

tests/cbindings/storage.c

@@ -1,8 +1,11 @@
+#include <errno.h>
+#include <pthread.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <stdint.h>
 #include <stdbool.h>
+#include <time.h>
 #include <unistd.h>
 #include "../../library/libstorage.h"
 
@@ -51,6 +54,9 @@
 
 typedef struct
 {
+    pthread_mutex_t mutex;
+    pthread_cond_t cond;
+    bool done;
     int ret;
     char *msg;
     char *chunk;
@@ -60,6 +66,9 @@ typedef struct
 static Resp *alloc_resp(void)
 {
     Resp *r = (Resp *)calloc(1, sizeof(Resp));
+    pthread_mutex_init(&r->mutex, NULL);
+    pthread_cond_init(&r->cond, NULL);
+    r->done = false;
     r->msg = NULL;
     r->chunk = NULL;
     r->ret = -1;
@@ -83,6 +92,8 @@ static void free_resp(Resp *r)
         free(r->chunk);
     }
 
+    pthread_cond_destroy(&r->cond);
+    pthread_mutex_destroy(&r->mutex);
     free(r);
 }
 
@@ -93,7 +104,11 @@ static int get_ret(Resp *r)
         return RET_ERR;
     }
 
-    return r->ret;
+    pthread_mutex_lock(&r->mutex);
+    int ret = r->ret;
+    pthread_mutex_unlock(&r->mutex);
+
+    return ret;
 }
 
 // wait_resp waits until the async response is ready or max retries is reached.
@@ -101,13 +116,33 @@ static int get_ret(Resp *r)
 // indicate that the response is ready to be consumed.
 static void wait_resp(Resp *r)
 {
-    int retries = 0;
-
-    while (get_ret(r) == -1 && retries < MAX_RETRIES)
+    if (!r)
     {
-        usleep(1000 * 100); // 100 ms
-        retries++;
+        return;
     }
+
+    const long timeout_ms = MAX_RETRIES * 100;
+    struct timespec deadline;
+
+    clock_gettime(CLOCK_REALTIME, &deadline);
+    deadline.tv_sec += timeout_ms / 1000;
+    deadline.tv_nsec += (timeout_ms % 1000) * 1000000;
+    if (deadline.tv_nsec >= 1000000000)
+    {
+        deadline.tv_sec += 1;
+        deadline.tv_nsec -= 1000000000;
+    }
+
+    pthread_mutex_lock(&r->mutex);
+    while (!r->done)
+    {
+        int rc = pthread_cond_timedwait(&r->cond, &r->mutex, &deadline);
+        if (rc == ETIMEDOUT)
+        {
+            break;
+        }
+    }
+    pthread_mutex_unlock(&r->mutex);
 }
 
 // is_resp_ok checks if the async response indicates success.
@@ -122,6 +157,8 @@ static int is_resp_ok(Resp *r, char **res)
 
     wait_resp(r);
 
+    pthread_mutex_lock(&r->mutex);
+
     int ret = (r->ret == RET_OK) ? RET_OK : RET_ERR;
 
     // If a response pointer is provided, it’s safe to initialize it to NULL.
@@ -142,6 +179,8 @@ static int is_resp_ok(Resp *r, char **res)
         *res = strdup(r->msg);
     }
 
+    pthread_mutex_unlock(&r->mutex);
+
     free_resp(r);
 
     return ret;
@@ -168,8 +207,7 @@ static void callback(int ret, const char *msg, size_t len, void *userData)
         return;
     }
 
-    // Assign the return code to the response structure.
-    r->ret = ret;
+    pthread_mutex_lock(&r->mutex);
 
     // If the reponse already has a message, just free it first.
     if (r->msg)
@@ -187,8 +225,8 @@ static void callback(int ret, const char *msg, size_t len, void *userData)
         r->len = len;
     }
 
-    // For other cases, copy the message data.
-    if (msg && len > 0)
+    // For terminal responses, copy the message data.
+    if (ret != RET_PROGRESS && msg && len > 0)
     {
         // Allocate memory for the message plus null terminator.
         r->msg = (char *)malloc(len + 1);
@@ -197,6 +235,10 @@ static void callback(int ret, const char *msg, size_t len, void *userData)
         if (!r->msg)
         {
             r->len = 0;
+            r->ret = RET_ERR;
+            r->done = true;
+            pthread_cond_signal(&r->cond);
+            pthread_mutex_unlock(&r->mutex);
             return;
         }
 
@@ -208,11 +250,25 @@ static void callback(int ret, const char *msg, size_t len, void *userData)
 
         r->len = len;
     }
-    else
+    else if (ret != RET_PROGRESS)
     {
         r->msg = NULL;
         r->len = 0;
     }
+
+    // Progress updates are intermediate callbacks. Keep any copied chunk data,
+    // but wait for the final RET_OK/RET_ERR before completing the response.
+    if (ret == RET_PROGRESS)
+    {
+        pthread_mutex_unlock(&r->mutex);
+        return;
+    }
+
+    // Publish completion last so wait_resp can only observe a fully written Resp.
+    r->ret = ret;
+    r->done = true;
+    pthread_cond_signal(&r->cond);
+    pthread_mutex_unlock(&r->mutex);
 }
 
 static int read_file(const char *filepath, char **res)
@@ -256,7 +312,7 @@ int setup(void **storage_ctx)
 
     wait_resp(r);
 
-    if (r->ret != RET_OK)
+    if (get_ret(r) != RET_OK)
     {
         free_resp(r);
         return RET_ERR;
@@ -343,9 +399,9 @@ int check_repo(void *storage_ctx)
 
     int ret = is_resp_ok(r, &res);
 
-    if (strcmp(res, "./data-dir") != 0)
+    if (res == NULL || strcmp(res, "./data-dir") != 0)
     {
-        printf("repo mismatch: %s\n", res);
+        printf("repo mismatch: %s\n", res ? res : "(null)");
         ret = RET_ERR;
     }
 
@@ -368,9 +424,9 @@ int check_debug(void *storage_ctx)
     int ret = is_resp_ok(r, &res);
 
     // Simple check to ensure the response contains spr
-    if (strstr(res, "spr") == NULL)
+    if (res == NULL || strstr(res, "spr") == NULL)
     {
-        fprintf(stderr, "debug content mismatch, res:%s\n", res);
+        fprintf(stderr, "debug content mismatch, res:%s\n", res ? res : "(null)");
         ret = RET_ERR;
     }
 
@@ -392,9 +448,9 @@ int check_spr(void *storage_ctx)
 
     int ret = is_resp_ok(r, &res);
 
-    if (strstr(res, "spr") == NULL)
+    if (res == NULL || strstr(res, "spr") == NULL)
     {
-        fprintf(stderr, "spr content mismatch, res:%s\n", res);
+        fprintf(stderr, "spr content mismatch, res:%s\n", res ? res : "(null)");
         ret = RET_ERR;
     }
 
@@ -562,7 +618,7 @@ int check_upload_file(void *storage_ctx, const char *filepath, char **res)
 
     int ret = is_resp_ok(r, res);
 
-    if (res == NULL || strlen(*res) == 0)
+    if (res == NULL || *res == NULL || strlen(*res) == 0)
     {
         fprintf(stderr, "CID is missing\n");
         return RET_ERR;
@@ -600,9 +656,9 @@ int check_download_stream(void *storage_ctx, const char *cid, const char *filepa
 
     int ret = is_resp_ok(r, &res);
 
-    if (strncmp(res, "Hello World!", strlen("Hello World!")) != 0)
+    if (res == NULL || strncmp(res, "Hello World!", strlen("Hello World!")) != 0)
     {
-        fprintf(stderr, "downloaded content mismatch, res:%s\n", res);
+        fprintf(stderr, "downloaded content mismatch, res:%s\n", res ? res : "(null)");
         ret = RET_ERR;
     }
 
@@ -612,9 +668,9 @@ int check_download_stream(void *storage_ctx, const char *cid, const char *filepa
         ret = RET_ERR;
     }
 
-    if (strncmp(res, "Hello World!", strlen("Hello World!")) != 0)
+    if (res == NULL || strncmp(res, "Hello World!", strlen("Hello World!")) != 0)
     {
-        fprintf(stderr, "downloaded content mismatch, res:%s\n", res);
+        fprintf(stderr, "downloaded content mismatch, res:%s\n", res ? res : "(null)");
         ret = RET_ERR;
     }
 
@@ -652,9 +708,9 @@ int check_download_chunk(void *storage_ctx, const char *cid)
 
     int ret = is_resp_ok(r, &res);
 
-    if (strncmp(res, "Hello World!", strlen("Hello World!")) != 0)
+    if (res == NULL || strncmp(res, "Hello World!", strlen("Hello World!")) != 0)
     {
-        fprintf(stderr, "downloaded chunk content mismatch, res:%s\n", res);
+        fprintf(stderr, "downloaded chunk content mismatch, res:%s\n", res ? res : "(null)");
         ret = RET_ERR;
     }
 
@@ -691,9 +747,9 @@ int check_download_manifest(void *storage_ctx, const char *cid)
 
     const char *expected_manifest = "{\"manifestVersion\":0,\"treeCid\":\"zDzSvJTf8JYwvysKPmG7BtzpbiAHfuwFMRphxm4hdvnMJ4XPJjKX\",\"datasetSize\":12,\"blockSize\":65536,\"filename\":\"hello_world.txt\",\"mimetype\":\"text/plain\"}";
 
-    if (strncmp(res, expected_manifest, strlen(expected_manifest)) != 0)
+    if (res == NULL || strncmp(res, expected_manifest, strlen(expected_manifest)) != 0)
     {
-        fprintf(stderr, "downloaded manifest content mismatch, res:%s\n", res);
+        fprintf(stderr, "downloaded manifest content mismatch, res:%s\n", res ? res : "(null)");
         ret = RET_ERR;
     }
 
@@ -717,9 +773,9 @@ int check_list(void *storage_ctx)
 
     const char *expected_manifest = "{\"manifestVersion\":0,\"treeCid\":\"zDzSvJTf8JYwvysKPmG7BtzpbiAHfuwFMRphxm4hdvnMJ4XPJjKX\",\"datasetSize\":12,\"blockSize\":65536,\"filename\":\"hello_world.txt\",\"mimetype\":\"text/plain\"}";
 
-    if (strstr(res, expected_manifest) == NULL)
+    if (res == NULL || strstr(res, expected_manifest) == NULL)
     {
-        fprintf(stderr, "downloaded manifest content mismatch, res:%s\n", res);
+        fprintf(stderr, "downloaded manifest content mismatch, res:%s\n", res ? res : "(null)");
         ret = RET_ERR;
     }
 
@@ -742,9 +798,9 @@ int check_space(void *storage_ctx)
     int ret = is_resp_ok(r, &res);
 
     // Simple check to ensure the response contains totalBlocks
-    if (strstr(res, "totalBlocks") == NULL)
+    if (res == NULL || strstr(res, "totalBlocks") == NULL)
     {
-        fprintf(stderr, "list content mismatch, res:%s\n", res);
+        fprintf(stderr, "space content mismatch, res:%s\n", res ? res : "(null)");
         ret = RET_ERR;
     }
 
@@ -768,17 +824,17 @@ int check_exists(void *storage_ctx, const char *cid, bool expected)
 
     if (expected)
     {
-        if (strcmp(res, "true") != 0)
+        if (res == NULL || strcmp(res, "true") != 0)
         {
-            fprintf(stderr, "exists content mismatch, res:%s\n", res);
+            fprintf(stderr, "exists content mismatch, res:%s\n", res ? res : "(null)");
             ret = RET_ERR;
         }
     }
     else
     {
-        if (strcmp(res, "false") != 0)
+        if (res == NULL || strcmp(res, "false") != 0)
         {
-            fprintf(stderr, "exists content mismatch, res:%s\n", res);
+            fprintf(stderr, "exists content mismatch, res:%s\n", res ? res : "(null)");
             ret = RET_ERR;
         }
     }
@@ -813,9 +869,9 @@ int check_toggle_private_queries(void *storage_ctx)
     }
 
     int ret = is_resp_ok(r, &res);
-    if (strcmp(res, "false") != 0)
+    if (res == NULL || strcmp(res, "false") != 0)
     {
-        fprintf(stderr, "toggle private queries content mismatch, res:%s\n", res);
+        fprintf(stderr, "toggle private queries content mismatch, res:%s\n", res ? res : "(null)");
         free(res);
         return RET_ERR;
     }
@@ -830,9 +886,39 @@ int check_toggle_private_queries(void *storage_ctx)
     }
 
     ret = is_resp_ok(r, &res);
-    if (strcmp(res, "true") != 0)
+    if (res == NULL || strcmp(res, "true") != 0)
     {
-        fprintf(stderr, "toggle private queries content mismatch, res:%s\n", res);
+        fprintf(stderr, "toggle private queries content mismatch, res:%s\n", res ? res : "(null)");
+        free(res);
+        return RET_ERR;
+    }
+
+    free(res);
+    return RET_OK;
+}
+
+int check_get_metrics(void *storage_ctx)
+{
+    Resp *r = alloc_resp();
+    char *res = NULL;
+
+    if (storage_get_metrics(storage_ctx, (StorageCallback)callback, r) != RET_OK)
+    {
+        free_resp(r);
+        return RET_ERR;
+    }
+
+    int ret = is_resp_ok(r, &res);
+    if (ret != RET_OK)
+    {
+        free(res);
+        return ret;
+    }
+
+    // Checks that response contains a metric we are SURE must exist
+    if (res == NULL || strstr(res, "libp2p_successful_dials_total") == NULL)
+    {
+        fprintf(stderr, "get_metrics missing expected metric\n");
         free(res);
         return RET_ERR;
     }
@@ -894,7 +980,8 @@ int main(void)
 
     RUN_TEST(check_toggle_private_queries(storage_ctx));
     RUN_TEST(update_log_level(storage_ctx, "TRACE"));
+    RUN_TEST(check_get_metrics(storage_ctx));
     RUN_TEST(cleanup(storage_ctx));
 
     END_SUITE
-}
+}

tests/libstorage/logosmetrics.nim

@@ -0,0 +1,148 @@
+import std/json
+import std/times
+
+import pkg/unittest2
+import pkg/metrics
+
+import ../../library/logosmetrics
+
+declareCounter myCounter, "Parts Counter", ["part_type"]
+declareGauge myGauge, "My gauge"
+declareHistogram myHistogram, "My histogram", buckets = [0.0, 1.0, 2.0]
+
+# A badly-behaved collector that emits label/labelValue arrays of differing
+# lengths.
+type BadCollector = ref object of Gauge
+
+method collect(collector: BadCollector, output: MetricHandler) =
+  output(
+    name = "bad_metric",
+    value = 1.0,
+    labels = ["label1", "label2"],
+    labelValues = ["value1"],
+    timestamp = Time(),
+  )
+  output(
+    name = "bad_metric",
+    value = 1.0,
+    labels = ["label1", "label2"],
+    labelValues = ["value1", "value2"],
+    timestamp = Time(),
+  )
+
+suite "Metrics":
+  test "should serialize Nim metrics to Logos Metrics format":
+    myCounter.inc(labelValues = ["screws"])
+    myCounter.inc(labelValues = ["washers"])
+
+    myGauge.set(42.0)
+
+    myHistogram.observe(1)
+    myHistogram.observe(2)
+    myHistogram.observe(3)
+    myHistogram.observe(4)
+    myHistogram.observe(5)
+
+    let metrics =
+      defaultRegistry.toJson(includeOnly = @["myCounter", "myGauge", "myHistogram"])
+
+    # Remove "created" metrics as we can't pin those down.
+    var filteredMetrics = %*{"metrics": @[]}
+    for metric in metrics["metrics"]:
+      if metric["name"].getStr() != "myCounter_created" and
+          metric["name"].getStr() != "myHistogram_created":
+        filteredMetrics["metrics"].add(metric)
+
+    check filteredMetrics ==
+      %*{
+        "metrics": [
+          {
+            "name": "myCounter_total",
+            "type": "counter",
+            "help": "Parts Counter",
+            "value": 1.0,
+            "labels": {"part_type": "screws"},
+          },
+          {
+            "name": "myCounter_total",
+            "type": "counter",
+            "help": "Parts Counter",
+            "value": 1.0,
+            "labels": {"part_type": "washers"},
+          },
+          {
+            "name": "myGauge",
+            "type": "gauge",
+            "help": "My gauge",
+            "value": 42.0,
+            "labels": {},
+          },
+          {
+            "name": "myHistogram_sum",
+            "type": "histogram",
+            "help": "My histogram",
+            "value": 15.0,
+            "labels": {},
+          },
+          {
+            "name": "myHistogram_count",
+            "type": "histogram",
+            "help": "My histogram",
+            "value": 5.0,
+            "labels": {},
+          },
+          {
+            "name": "myHistogram_bucket",
+            "type": "histogram",
+            "help": "My histogram",
+            "value": 0.0,
+            "labels": {"le": "0.0"},
+          },
+          {
+            "name": "myHistogram_bucket",
+            "type": "histogram",
+            "help": "My histogram",
+            "value": 1.0,
+            "labels": {"le": "1.0"},
+          },
+          {
+            "name": "myHistogram_bucket",
+            "type": "histogram",
+            "help": "My histogram",
+            "value": 2.0,
+            "labels": {"le": "2.0"},
+          },
+          {
+            "name": "myHistogram_bucket",
+            "type": "histogram",
+            "help": "My histogram",
+            "value": 5.0,
+            "labels": {"le": "+Inf"},
+          },
+        ]
+      }
+
+  test "should drop labels when collector emits fewer values than labels":
+    discard BadCollector.newCollector("badMetric", "Badly behaved collector")
+
+    let metrics = defaultRegistry.toJson(includeOnly = @["badMetric"])
+
+    check metrics ==
+      %*{
+        "metrics": [
+          {
+            "name": "bad_metric",
+            "type": "gauge",
+            "help": "Badly behaved collector",
+            "value": 1.0,
+            "labels": {"label1": "value1"},
+          },
+          {
+            "name": "bad_metric",
+            "type": "gauge",
+            "help": "Badly behaved collector",
+            "value": 1.0,
+            "labels": {"label1": "value1", "label2": "value2"},
+          },
+        ]
+      }

tests/testLibstorage.nim

@@ -0,0 +1,4 @@
+# Tests the Nim side of libstorage.
+import ./libstorage/logosmetrics
+
+{.warning[UnusedImport]: off.}