all: Add Prometheus-style metrics to expose some internal performance counters (fixes #5175) (#9003)

lib/events/events.go

@@ -297,6 +297,7 @@ loop:
 		case e := <-l.events:
 			// Incoming events get sent
 			l.sendEvent(e)
+			metricEvents.WithLabelValues(e.Type.String(), metricEventStateCreated).Inc()
 
 		case fn := <-l.funcs:
 			// Subscriptions are handled here.
@@ -345,9 +346,11 @@ func (l *logger) sendEvent(e Event) {
 
 			select {
 			case s.events <- e:
+				metricEvents.WithLabelValues(e.Type.String(), metricEventStateDelivered).Inc()
 			case <-l.timeout.C:
 				// if s.events is not ready, drop the event
 				timedOut = true
+				metricEvents.WithLabelValues(e.Type.String(), metricEventStateDropped).Inc()
 			}
 
 			// If stop returns false it already sent something to the

lib/events/metrics.go

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package events
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+var metricEvents = promauto.NewCounterVec(prometheus.CounterOpts{
+	Namespace: "syncthing",
+	Subsystem: "events",
+	Name:      "total",
+	Help:      "Total number of created/forwarded/dropped events",
+}, []string{"event", "state"})
+
+const (
+	metricEventStateCreated   = "created"
+	metricEventStateDelivered = "delivered"
+	metricEventStateDropped   = "dropped"
+)