Add openmetrics and exemplars support (#544)

Co-authored-by: Andrei Dobre <andreidobre.web@gmail.com>

CHANGELOG.md

@@ -18,6 +18,8 @@ project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Added
 
+- Support for OpenMetrics and Exemplars
+
 ## [14.2.0] - 2023-03-06
 
 ### Changed

README.md

@@ -393,6 +393,47 @@ Default labels will be overridden if there is a name conflict.
 
 `register.clear()` will clear default labels.
 
+### Exemplars
+
+The exemplars defined in the OpenMetrics specification can be enabled on Counter
+and Histogram metric types. The default metrics have support for OpenTelemetry,
+they will populate the exemplars with the labels `{traceId, spanId}` and their
+corresponding values.
+
+The format for `inc()` and `observe()` calls are different if exemplars are
+enabled. They get a single object with the format
+`{labels, value, exemplarLabels}`.
+
+When using exemplars, the registry used for metrics should be set to OpenMetrics
+type (including the global or default registry if no registries are specified).
+
+### Registy type
+
+The library supports both the old Prometheus format and the OpenMetrics format.
+The format can be set per registry. For default metrics:
+
+```js
+const Prometheus = require('prom-client');
+Prometheus.register.setContentType(
+  Prometheus.Registry.OPENMETRICS_CONTENT_TYPE,
+);
+```
+
+Currently available registry types are defined by the content types:
+
+**PROMETHEUS_CONTENT_TYPE** - version 0.0.4 of the original Prometheus metrics,
+this is currently the default registry type.
+
+**OPENMETRICS_CONTENT_TYPE** - defaults to version 1.0.0 of the
+[OpenMetrics standard](https://github.com/OpenObservability/OpenMetrics/blob/d99b705f611b75fec8f450b05e344e02eea6921d/specification/OpenMetrics.md).
+
+The HTTP Content-Type string for each registry type is exposed both at module
+level (`prometheusContentType` and `openMetricsContentType`) and as static
+properties on the `Registry` object.
+
+The `contentType` constant exposed by the module returns the default content
+type when creating a new registry, currently defaults to Prometheus type.
+
 ### Multiple registries
 
 By default, metrics are automatically registered to the global registry (located
@@ -407,6 +448,9 @@ Registry has a `merge` function that enables you to expose multiple registries
 on the same endpoint. If the same metric name exists in both registries, an
 error will be thrown.
 
+Merging registries of different types is undefined. The user needs to make sure
+all used registries have the same type (Prometheus or OpenMetrics versions).
+
 ```js
 const client = require('prom-client');
 const registry = new client.Registry();
@@ -557,9 +601,6 @@ new client.Histogram({
 });
 ```
 
-The content-type prometheus expects is also exported as a constant, both on the
-`register` and from the main file of this project, called `contentType`.
-
 ### Garbage Collection Metrics
 
 To avoid native dependencies in this module, GC statistics for bytes reclaimed

example/exemplars.js

@@ -0,0 +1,89 @@
+'use strict';
+
+const { register, Registry, Counter, Histogram } = require('..');
+
+async function makeCounters() {
+	const c = new Counter({
+		name: 'test_counter_exemplar',
+		help: 'Example of a counter with exemplar',
+		labelNames: ['code'],
+		enableExemplars: true,
+	});
+
+	const exemplarLabels = { traceId: '888', spanId: 'jjj' };
+
+	c.inc({
+		labels: { code: 300 },
+		value: 1,
+		exemplarLabels,
+	});
+	c.inc({
+		labels: { code: 200 },
+		exemplarLabels,
+	});
+
+	c.inc({ exemplarLabels });
+	c.inc();
+}
+
+async function makeHistograms() {
+	const h = new Histogram({
+		name: 'test_histogram_exemplar',
+		help: 'Example of a histogram with exemplar',
+		labelNames: ['code'],
+		enableExemplars: true,
+	});
+
+	const exemplarLabels = { traceId: '111', spanId: 'zzz' };
+
+	h.observe({
+		labels: { code: '200' },
+		value: 1,
+		exemplarLabels,
+	});
+
+	h.observe({
+		labels: { code: '200' },
+		value: 3,
+		exemplarLabels,
+	});
+
+	h.observe({
+		labels: { code: '200' },
+		value: 0.3,
+		exemplarLabels,
+	});
+
+	h.observe({
+		labels: { code: '200' },
+		value: 300,
+		exemplarLabels,
+	});
+}
+
+async function main() {
+	// exemplars will be shown only by OpenMetrics registry types
+	register.setContentType(Registry.OPENMETRICS_CONTENT_TYPE);
+
+	makeCounters();
+	makeHistograms();
+
+	console.log(await register.metrics());
+	console.log('---');
+
+	// if you dont want to set the default registry to OpenMetrics type then you need to create a new registry and assign it to the metric
+
+	register.setContentType(Registry.PROMETHEUS_CONTENT_TYPE);
+	const omReg = new Registry(Registry.OPENMETRICS_CONTENT_TYPE);
+	const c = new Counter({
+		name: 'counter_with_exemplar',
+		help: 'Example of a counter',
+		labelNames: ['code'],
+		registers: [omReg],
+		enableExemplars: true,
+	});
+	c.inc({ labels: { code: '200' }, exemplarLabels: { traceId: 'traceA' } });
+	console.log(await omReg.metrics());
+}
+
+main();

index.d.ts

@@ -1,10 +1,27 @@
 // Type definitions for prom-client
 // Definitions by: Simon Nyberg http://twitter.com/siimon_nyberg
 
+export type Charset = 'utf-8';
+
+export type PrometheusMIME = 'text/plain';
+export type PrometheusMetricsVersion = '0.0.4';
+
+export type OpenMetricsMIME = 'application/openmetrics-text';
+export type OpenMetricsVersion = '1.0.0';
+
+export type PrometheusContentType =
+	`${OpenMetricsMIME}; version=${OpenMetricsVersion}; charset=${Charset}`;
+export type OpenMetricsContentType =
+	`${PrometheusMIME}; version=${PrometheusMetricsVersion}; charset=${Charset}`;
+
+export type RegistryContentType =
+	| PrometheusContentType
+	| OpenMetricsContentType;
+
 /**
  * Container for all registered metrics
  */
-export class Registry {
+export class Registry<RegistryContentType = PrometheusContentType> {
 	/**
 	 * Get string representation for all metrics
 	 */
@@ -64,7 +81,14 @@ export class Registry {
 	/**
 	 * Gets the Content-Type of the metrics for use in the response headers.
 	 */
-	contentType: string;
+	contentType: RegistryContentType;
+
+	/**
+	 * Set the content type of a registry. Used to change between Prometheus and
+	 * OpenMetrics versions.
+	 * @param contentType The type of the registry
+	 */
+	setContentType(contentType: RegistryContentType): void;
 
 	/**
 	 * Merge registers
@@ -80,9 +104,20 @@ export type Collector = () => void;
 export const register: Registry;
 
 /**
- * The Content-Type of the metrics for use in the response headers.
+ * HTTP Content-Type for metrics response headers, defaults to Prometheus text
+ * format.
+ */
+export const contentType: RegistryContentType;
+
+/**
+ * HTTP Prometheus Content-Type for metrics response headers.
  */
-export const contentType: string;
+export const prometheusContentType: PrometheusContentType;
+
+/**
+ * HTTP OpenMetrics Content-Type for metrics response headers.
+ */
+export const openMetricsContentType: OpenMetricsContentType;
 
 export class AggregatorRegistry extends Registry {
 	/**
@@ -164,16 +199,32 @@ interface MetricConfiguration<T extends string> {
 	name: string;
 	help: string;
 	labelNames?: T[] | readonly T[];
-	registers?: Registry[];
+	registers?: (
+		| Registry<PrometheusContentType>
+		| Registry<OpenMetricsContentType>
+	)[];
 	aggregator?: Aggregator;
 	collect?: CollectFunction<any>;
+	enableExemplars?: boolean;
 }
 
 export interface CounterConfiguration<T extends string>
 	extends MetricConfiguration<T> {
 	collect?: CollectFunction<Counter<T>>;
 }
 
+export interface IncreaseDataWithExemplar<T extends string> {
+	value?: number;
+	labels?: LabelValues<T>;
+	exemplarLabels?: LabelValues<T>;
+}
+
+export interface ObserveDataWithExemplar<T extends string> {
+	value: number;
+	labels?: LabelValues<T>;
+	exemplarLabels?: LabelValues<T>;
+}
+
 /**
  * A counter is a cumulative metric that represents a single numerical value that only ever goes up
  */
@@ -196,6 +247,12 @@ export class Counter<T extends string = string> {
 	 */
 	inc(value?: number): void;
 
+	/**
+	 * Increment with exemplars
+	 * @param incData Object with labels, value and exemplars for an increase
+	 */
+	inc(incData: IncreaseDataWithExemplar<T>): void;
+
 	/**
 	 * Get counter metric object
 	 */
@@ -410,6 +467,12 @@ export class Histogram<T extends string = string> {
 	 */
 	observe(labels: LabelValues<T>, value: number): void;
 
+	/**
+	 * Observe with exemplars
+	 * @param observeData Object with labels, value and exemplars for an observation
+	 */
+	observe(observeData: ObserveDataWithExemplar<T>): void;
+
 	/**
 	 * Get histogram metric object
 	 */

index.js

@@ -8,6 +8,10 @@
 exports.register = require('./lib/registry').globalRegistry;
 exports.Registry = require('./lib/registry');
 exports.contentType = require('./lib/registry').globalRegistry.contentType;
+exports.prometheusContentType =
+	require('./lib/registry').PROMETHEUS_CONTENT_TYPE;
+exports.openMetricsContentType =
+	require('./lib/registry').OPENMETRICS_CONTENT_TYPE;
 exports.validateMetricName = require('./lib/validation').validateMetricName;
 
 exports.Counter = require('./lib/counter');

lib/cluster.js

@@ -28,8 +28,8 @@ let listenersAdded = false;
 const requests = new Map(); // Pending requests for workers' local metrics.
 
 class AggregatorRegistry extends Registry {
-	constructor() {
-		super();
+	constructor(regContentType = Registry.PROMETHEUS_CONTENT_TYPE) {
+		super(regContentType);
 		addListeners();
 	}
 
@@ -84,19 +84,30 @@ class AggregatorRegistry extends Registry {
 		});
 	}
 
+	get contentType() {
+		return super.contentType;
+	}
+
 	/**
 	 * Creates a new Registry instance from an array of metrics that were
 	 * created by `registry.getMetricsAsJSON()`. Metrics are aggregated using
 	 * the method specified by their `aggregator` property, or by summation if
 	 * `aggregator` is undefined.
 	 * @param {Array} metricsArr Array of metrics, each of which created by
 	 *   `registry.getMetricsAsJSON()`.
+	 * @param {string} registryType content type of the new registry. Defaults
+	 * to PROMETHEUS_CONTENT_TYPE.
 	 * @return {Registry} aggregated registry.
 	 */
-	static aggregate(metricsArr) {
+	static aggregate(
+		metricsArr,
+		registryType = Registry.PROMETHEUS_CONTENT_TYPE,
+	) {
 		const aggregatedRegistry = new Registry();
 		const metricsByName = new Grouper();
 
+		aggregatedRegistry.setContentType(registryType);
+
 		// Gather by name
 		metricsArr.forEach(metrics => {
 			metrics.forEach(metric => {