eth/protocols, metrics: use resetting histograms for rare packets

eth/protocols/eth/handler.go

@@ -246,7 +246,11 @@ func handleMessage(backend Backend, peer *Peer) error {
 	if metrics.Enabled {
 		h := fmt.Sprintf("%s/%s/%d/%#02x", p2p.HandleHistName, ProtocolName, peer.Version(), msg.Code)
 		defer func(start time.Time) {
-			sampler := func() metrics.Sample { return metrics.NewExpDecaySample(1028, 0.015) }
+			sampler := func() metrics.Sample {
+				return metrics.ResettingSample(
+					metrics.NewExpDecaySample(1028, 0.015),
+				)
+			}
 			metrics.GetOrRegisterHistogramLazy(h, nil, sampler).Update(time.Since(start).Microseconds())
 		}(time.Now())
 	}

eth/protocols/snap/handler.go

@@ -134,7 +134,11 @@ func handleMessage(backend Backend, peer *Peer) error {
 	if metrics.Enabled {
 		h := fmt.Sprintf("%s/%s/%d/%#02x", p2p.HandleHistName, ProtocolName, peer.Version(), msg.Code)
 		defer func(start time.Time) {
-			sampler := func() metrics.Sample { return metrics.NewExpDecaySample(1028, 0.015) }
+			sampler := func() metrics.Sample {
+				return metrics.ResettingSample(
+					metrics.NewExpDecaySample(1028, 0.015),
+				)
+			}
 			metrics.GetOrRegisterHistogramLazy(h, nil, sampler).Update(time.Since(start).Microseconds())
 		}(time.Now())
 	}

metrics/resetting_sample.go

@@ -0,0 +1,24 @@
+package metrics
+
+// ResettingSample converts an ordinary sample into one that resets whenever its
+// snapshot is retrieved. This will break for multi-monitor systems, but when only
+// a single metric is being pushed out, this ensure that low-frequency events don't
+// skew th charts indefinitely.
+func ResettingSample(sample Sample) Sample {
+	return &resettingSample{
+		Sample: sample,
+	}
+}
+
+// resettingSample is a simple wrapper around a sample that resets it upon the
+// snapshot retrieval.
+type resettingSample struct {
+	Sample
+}
+
+// Snapshot returns a read-only copy of the sample with the original reset.
+func (rs *resettingSample) Snapshot() Sample {
+	s := rs.Sample.Snapshot()
+	rs.Sample.Clear()
+	return s
+}