metric: add noop metrics.

Add a noop type of metric not registered by service-runner Metric interface.
This is used to delegate collection of default prometheus
metrics to prom-client.collectDefaultMetrics.

lib/heapwatch.js

@@ -4,6 +4,16 @@ const cluster = require( 'cluster' );
 
 class HeapWatch {
 	constructor( conf, logger, metrics ) {
+		if ( metrics !== null ) {
+			const prometheusOptions = metrics.options.find( ( option ) =>
+				option.type === 'prometheus' && option.collect_default === true
+			);
+			// When true, a noop dummy metric will be instantiated
+			// and passed down to a PrometheusClient instance.
+			// This metric won't be registered directly by service-runner,
+			// but will be used to trigger a call to prom-client.collectDefaultMetrics().
+			this.collect_default = !!prometheusOptions;
+		}
 		this.conf = conf = conf || {};
 		this.limit = ( conf.limitMB || 1500 ) * 1024 * 1024;
 		this.logger = logger;
@@ -15,6 +25,17 @@ class HeapWatch {
 
 	watch() {
 		const usage = process.memoryUsage();
+		if ( this.collect_default ) {
+			// collect some default metrics recommended by Prometheus itself.
+			// https://prometheus.io/docs/instrumenting/writing_clientlibs/#standard-and-runtime-collectors
+			this.metrics.makeMetric( {
+				type: 'noop',
+				name: 'prometheus.default',
+				prometheus: {
+					collect_default: true
+				}
+			} );
+		}
 		this.metrics.makeMetric( {
 			type: 'Gauge',
 			name: 'heap.rss',

lib/metrics/index.js

@@ -88,12 +88,17 @@ class Metrics {
 	//     }
 	// }
 	makeMetric( options ) {
-		let metric = this.cache.get( options.name );
-		if ( metric === undefined ) {
-			metric = new Metric( this.clients, this.logger, options );
-			this.cache.set( options.name, metric );
+		// noop is a kind of metric not registered by service-runner Metric interface.
+		// This is used to delegate collection of default prometheus
+		// metrics to prom-client.collectDefaultMetrics.
+		if ( options.collect_default === undefined || options.type !== 'noop' ) {
+			let metric = this.cache.get( options.name );
+			if ( metric === undefined ) {
+				metric = new Metric( this.clients, this.logger, options );
+				this.cache.set( options.name, metric );
+			}
+			return metric;
 		}
-		return metric;
 	}
 
 	close() {

lib/metrics/metric.js

@@ -81,6 +81,7 @@ class Metric {
 			this.logger.log( 'error/metrics', `endTiming() unsupported for metric type ${ this.type }` );
 		}
 	}
+
 }
 
 module.exports = Metric;

lib/metrics/prometheus.js

@@ -8,11 +8,6 @@ class PrometheusMetric {
 	constructor( options, client ) {
 		this.client = client;
 		this.options = cloneDeep( options );
-
-		// TODO: update eslint rules to allow optional chaining operator `?`
-		if ( this.options.prometheus && this.options.prometheus.collect_default === true ) {
-			this.client.collectDefaultMetrics();
-		}
 		if ( this.options.labels === undefined ) {
 			this.options.labels = { names: [] };
 		}
@@ -26,13 +21,24 @@ class PrometheusMetric {
 		this.options.labels.names = this.options.labels.names.map( this._normalize );
 
 		this.options.prometheus.name = this._normalize( this.options.prometheus.name );
-		this.metric = new this.client[ this.options.type ]( {
-			name: this.options.prometheus.name,
-			help: this.options.prometheus.help,
-			labelNames: this.options.labels.names,
-			buckets: this.options.prometheus.buckets,
-			percentiles: this.options.prometheus.percentiles
-		} );
+		if ( this.options.type !== 'noop' ) {
+			this.metric = new this.client[ this.options.type ]( {
+				name: this.options.prometheus.name,
+				help: this.options.prometheus.help,
+				labelNames: this.options.labels.names,
+				buckets: this.options.prometheus.buckets,
+				percentiles: this.options.prometheus.percentiles
+			} );
+		} else if ( this.options.prometheus && this.options.prometheus.collect_default === true ) {
+			// TODO: update eslint rules to allow optional chaining operator `?`
+			// A no op metric.
+			// Invoke collectDefaultMetrics() but don't register any new metric
+			// via the service-runner Metric interface.
+			// https://prometheus.io/docs/instrumenting/writing_clientlibs/#standard-and-runtime-collectors
+			// TODO: collectDefaultMetrics() does not support providing labels.
+			//  Default labels should be set for the whole registry.
+			this.client.collectDefaultMetrics();
+		}
 	}
 
 	/**

test/features/tests.js

@@ -103,6 +103,28 @@ describe( 'service-runner tests', () => {
 			.finally( () => process.removeListener( 'warning', warningListener ) );
 	} );
 
+	it( 'Must produce prometheus default metrics when hit ', () => {
+		const server = new TestServer( `${__dirname}/../utils/simple_config_no_workers_collect_default.yaml` );
+		const response = { status: null, body: null };
+		return server.start()
+			.then( () => {
+				preq.get( { uri: 'http://127.0.0.1:9000' } )
+					.then( ( res ) => {
+						response.status = res.status;
+						response.body = res.body;
+					} );
+			} )
+			.delay( 1000 )
+			.then( () => {
+				// nodejs_version_info is reported by calls to prom-client.collectDefaultMetrics()
+				assert.ok(
+					response.body.includes( 'nodejs_version_info ' ),
+					'Must collect default metrics prometheus output.'
+				);
+			} )
+			.finally( () => server.stop() );
+	} );
+
 	// preq prevents the AssertionErrors from surfacing and failing the test
 	// performing the test this way presents them correctly
 	it( 'Must increment hitcount metrics when hit, no workers', () => {

test/utils/simple_config_no_workers.yaml

@@ -4,7 +4,6 @@ logging:
 metrics:
   - type: prometheus
     port: 9000
-    collect_default: true
   - type: statsd
     host: localhost
     port: 8125

test/utils/simple_config_no_workers_collect_default.yaml

@@ -0,0 +1,15 @@
+num_workers: 0
+logging:
+  level: fatal
+metrics:
+  - type: prometheus
+    port: 9000
+    collect_default:  true
+  - type: statsd
+    host: localhost
+    port: 8125
+services:
+  - name: test
+    module: test/utils/simple_server.js
+    conf:
+      port: 12345