allow the metrics call to fail

Starting from 1.9.0 Geth removed the `debug_metrics` RPC call

Signed-off-by: Jakub Sokołowski <jakub@status.im>

README.md

@@ -1,3 +1,5 @@
+# WARNING: This software is deprecated since Geth `1.9.0` which introduced its own [Prometheus endpoint](https://blog.ethereum.org/2019/07/10/geth-v1-9-0/#metrics-collection).
+
 # geth_exporter
 
 `geth_exporter` is a metrics exporter for [Prometheus](https://github.com/prometheus/prometheus).

collector.go

@@ -39,19 +39,23 @@ func (c *collector) collect() (flatMetrics, error) {
 
 	m, err := cl.metrics()
 	if err != nil {
-		return nil, err
+		/* not all geth nodes have debug enabled, s might be nil */
+		log.Printf("failed to get metrics: %v", err)
 	}
 
+	/* can handle m being nil, will just return an empty flatMetrics */
 	all := transformMetrics(m)
 
-	/* optional syncing stats */
 	s, err := cl.syncingMetrics()
-	if err == nil {
+	if err != nil {
+		/* not all geth nodes have eth enabled, s might be nil */
+		log.Printf("failed to get syncing stats: %v", err)
+	}
+
 	sync := decodeSyncData(s, "sync_")
 	for k, v := range sync {
 		all[k] = v
 	}
-	}
 
 	for k := range all {
 		if !c.matchAllFilters(k) {

metrics.go

@@ -34,6 +34,8 @@ func transformMetrics(data metrics) flatMetrics {
 	return flattenMetrics(data, make(flatMetrics), "")
 }
 
+/* flattenMetrics can handle data being nil, it will just return memo.
+ * A range over a nil slice is like a range over an empty slice. */
 func flattenMetrics(data metrics, memo flatMetrics, prefix string) flatMetrics {
 	for k, v := range data {
 		key := prefix + normalizeKey(k)