Add eventloop idle time metric

CHANGELOG.md

@@ -19,6 +19,8 @@ project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Added
 
+- Added: `nodejs_eventloop_idle_seconds_total` metric to 'collectDefaultMetrics()`.
+
 ## [14.0.0] - 2021-09-18
 
 ### Breaking

lib/defaultMetrics.js

@@ -9,6 +9,7 @@ const osMemoryHeap = require('./metrics/osMemoryHeap');
 const processOpenFileDescriptors = require('./metrics/processOpenFileDescriptors');
 const processMaxFileDescriptors = require('./metrics/processMaxFileDescriptors');
 const eventLoopLag = require('./metrics/eventLoopLag');
+const eventLoopIdle = require('./metrics/eventLoopIdle');
 const processHandles = require('./metrics/processHandles');
 const processRequests = require('./metrics/processRequests');
 const heapSizeAndUsed = require('./metrics/heapSizeAndUsed');
@@ -23,6 +24,7 @@ const metrics = {
 	processOpenFileDescriptors,
 	processMaxFileDescriptors,
 	eventLoopLag,
+	eventLoopIdle,
 	processHandles,
 	processRequests,
 	heapSizeAndUsed,

lib/metrics/eventLoopIdle.js

@@ -0,0 +1,44 @@
+'use strict';
+
+const Counter = require('../counter');
+
+// Check if metric is available
+let nodeTiming;
+try {
+	/* eslint-disable node/no-unsupported-features/node-builtins */
+	nodeTiming = require('perf_hooks').performance.nodeTiming;
+} catch (ex) {
+	// node version is too old
+}
+
+// Reported only when perf_hooks.performance.nodeTiming.idleTime is available.
+const NODEJS_EVENTLOOP_IDLE = 'nodejs_eventloop_idle_seconds_total';
+
+module.exports = (registry, config = {}) => {
+	if (!nodeTiming || nodeTiming.idleTime === undefined) {
+		return;
+	}
+
+	const namePrefix = config.prefix ? config.prefix : '';
+	const labels = config.labels ? config.labels : {};
+	const labelNames = Object.keys(labels);
+	const registers = registry ? [registry] : undefined;
+
+	let lastIdle = nodeTiming.idleTime;
+
+	const idle = new Counter({
+		name: namePrefix + NODEJS_EVENTLOOP_IDLE,
+		help:
+			"Total amount of time the event loop has been idle within the event loop's event provider",
+		registers,
+		labelNames,
+		aggregator: 'average',
+		collect() {
+			const val = nodeTiming.idleTime / 1e3;
+			idle.inc(labels, val - lastIdle);
+			lastIdle = val;
+		},
+	});
+};
+
+module.exports.metricNames = [NODEJS_EVENTLOOP_IDLE];