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

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"
+)