feat(bench): add -d:msgPathCounters decode/hash instrumentation

Zero-cost message-path counters gated on -d:msgPathCounters. When the
define is absent, countDecode/countHash expand to discard: no globals,
no runtime cost, no behaviour change. Hooked into WakuMessage.decode
(codec) and computeMessageHash (digest), exported via waku_core. Adds
benchTopicValidator/benchTopicHandler accessors on WakuRelay (also
define-gated) so the message-path benchmark can replay the inbound
validator+handler decode path without a live network.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

logos_delivery/waku/waku_core/message.nim

@@ -1,3 +1,8 @@
-import ./message/message, ./message/default_values, ./message/codec, ./message/digest
+import
+  ./message/message,
+  ./message/default_values,
+  ./message/codec,
+  ./message/digest,
+  ./message/path_counters
 
-export message, default_values, codec, digest
+export message, default_values, codec, digest, path_counters

logos_delivery/waku/waku_core/message/codec.nim

@@ -4,7 +4,7 @@
 # - Proto definition: https://github.com/vacp2p/waku/blob/main/waku/message/v1/message.proto
 {.push raises: [].}
 
-import ../../common/protobuf, ../topics, ../time, ./message
+import ../../common/protobuf, ../topics, ../time, ./message, ./path_counters
 
 proc encode*(message: WakuMessage): ProtoBuffer =
   var buf = initProtoBuffer()
@@ -21,6 +21,7 @@ proc encode*(message: WakuMessage): ProtoBuffer =
   buf
 
 proc decode*(T: type WakuMessage, buffer: seq[byte]): ProtobufResult[T] =
+  countDecode(buffer.len)
   var msg = WakuMessage()
   let pb = initProtoBuffer(buffer)
 

logos_delivery/waku/waku_core/message/digest.nim

@@ -1,7 +1,7 @@
 {.push raises: [].}
 
 import std/sequtils, stew/[byteutils, endians2, arrayops], nimcrypto/sha2, results
-import ../topics, ./message
+import ../topics, ./message, ./path_counters
 
 ## 14/WAKU2-MESSAGE: Deterministic message hashing
 ## https://rfc.vac.dev/spec/14/#deterministic-message-hashing
@@ -51,6 +51,7 @@ proc hexToHash*(hexString: string): Result[WakuMessageHash, string] =
   return ok(hash)
 
 proc computeMessageHash*(pubsubTopic: PubsubTopic, msg: WakuMessage): WakuMessageHash =
+  countHash(msg.payload.len + msg.meta.len)
   var ctx: sha256
   ctx.init()
   defer:

logos_delivery/waku/waku_core/message/path_counters.nim

@@ -0,0 +1,40 @@
+## Message-path instrumentation counters (compile-time gated).
+##
+## Enabled only with `-d:msgPathCounters`. When the define is absent every hook
+## below expands to `discard` — no globals, no runtime cost, no behaviour change.
+## These counters exist purely so the message-path benchmark
+## (`apps/benchmarks/message_path_bench.nim`) can validate the decode/hash-per-
+## message claims in `docs/analysis/async_copy_analysis.md`.
+##
+## Single-threaded chronos assumption: plain `int64`, no atomics.
+
+{.push raises: [].}
+
+when defined(msgPathCounters):
+  type MsgPathCounters* = object
+    decodeCalls*: int64 ## `WakuMessage.decode` invocations
+    hashCalls*: int64 ## `computeMessageHash` invocations
+    decodedBytes*: int64 ## bytes fed into decode (proto buffer length)
+    hashedBytes*: int64 ## bytes fed into the hash (payload + meta)
+
+  var msgPathCounters* {.global.}: MsgPathCounters
+
+  template countDecode*(bytes: int) =
+    msgPathCounters.decodeCalls += 1
+    msgPathCounters.decodedBytes += int64(bytes)
+
+  template countHash*(bytes: int) =
+    msgPathCounters.hashCalls += 1
+    msgPathCounters.hashedBytes += int64(bytes)
+
+  proc resetMsgPathCounters*() =
+    msgPathCounters = MsgPathCounters()
+
+else:
+  template countDecode*(bytes: int) =
+    discard
+
+  template countHash*(bytes: int) =
+    discard
+
+{.pop.}

logos_delivery/waku/waku_relay/protocol.nim

@@ -633,6 +633,18 @@ proc subscribe*(w: WakuRelay, pubsubTopic: PubsubTopic, handler: WakuRelayHandle
   w.topicHealthDirty.incl(pubsubTopic)
   w.topicHealthUpdateEvent.fire()
 
+when defined(msgPathCounters):
+  ## Benchmark-only accessors (see `apps/benchmarks/message_path_bench.nim`).
+  ## They expose the per-topic gossipsub artifacts so the micro benchmark can
+  ## replay the inbound validator + handler decode path without a live network.
+  ## Guarded by `-d:msgPathCounters` so production builds never see them and the
+  ## `topicValidator`/`topicHandlers` fields stay module-private.
+  proc benchTopicValidator*(w: WakuRelay, pubsubTopic: PubsubTopic): ValidatorHandler =
+    w.topicValidator.getOrDefault(pubsubTopic)
+
+  proc benchTopicHandler*(w: WakuRelay, pubsubTopic: PubsubTopic): TopicHandler =
+    w.topicHandlers.getOrDefault(pubsubTopic)
+
 proc unsubscribeAll*(w: WakuRelay, pubsubTopic: PubsubTopic) =
   ## Unsubscribe all handlers on this pubsub topic