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

lib/scanner/metrics.go

@@ -0,0 +1,35 @@
+// 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 scanner
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+var (
+	metricHashedBytes = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "syncthing",
+		Subsystem: "scanner",
+		Name:      "hashed_bytes_total",
+		Help:      "Total amount of data hashed, per folder",
+	}, []string{"folder"})
+
+	metricScannedItems = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "syncthing",
+		Subsystem: "scanner",
+		Name:      "scanned_items_total",
+		Help:      "Total number of items (files/directories) inspected, per folder",
+	}, []string{"folder"})
+)
+
+func registerFolderMetrics(folderID string) {
+	// Register metrics for this folder, so that counters are present even
+	// when zero.
+	metricHashedBytes.WithLabelValues(folderID)
+	metricScannedItems.WithLabelValues(folderID)
+}