Add support for custom metrics

- Users can import `metricClient` function from openapm and use it any
of the files in app code to define custom metrics.
- This function internally uses prom-client's client so the same DSL
is supported.

playground/app.js

@@ -5,7 +5,11 @@
  *  */
 require('dotenv').config();
 const express = require('express');
-const { OpenAPM, setOpenAPMLabels } = require('../dist/src/index.js');
+const {
+  OpenAPM,
+  setOpenAPMLabels,
+  metricClient
+} = require('../dist/src/index.js');
 const mysql2 = require('mysql2');
 
 const openapm = new OpenAPM({
@@ -37,6 +41,12 @@ const pool = mysql2.createPool(
   'mysql://express-app:password@127.0.0.1/express' //  If this throws an error, Change the db url to the one you're running on your machine locally or the testing instance you might have hosted.
 );
 
+const client = metricClient();
+const counter = new client.Counter({
+  name: 'cancelation_calls',
+  help: 'no. of times cancel operation is called'
+});
+
 app.get('/result', (req, res) => {
   pool.getConnection((err, conn) => {
     conn.query(
@@ -59,6 +69,7 @@ app.get('/organizations/:org/users', (req, res) => {
 });
 
 app.get('/cancel/:ids', (req, res) => {
+  counter.inc();
   res.status(200).json({});
 });
 

src/OpenAPM.ts

@@ -471,4 +471,8 @@ export class OpenAPM extends LevitateEvents {
   }
 }
 
+export function metricClient() {
+  return promClient;
+}
+
 export default OpenAPM;

src/index.ts

@@ -1,3 +1,3 @@
-export { default as OpenAPM } from './OpenAPM';
+export { default as OpenAPM, metricClient } from './OpenAPM';
 export { setOpenAPMLabels } from './async-local-storage.http';
 export type { OpenAPMOptions } from './OpenAPM';

tests/express.test.ts

@@ -49,6 +49,15 @@ describe('REDMiddleware', () => {
     ).toBe(NUMBER_OF_REQUESTS);
   });
 
+  test('Captures Custom Counter Metrics - App', async () => {
+    expect(
+      parseInt(
+        parsedData?.find((m) => m.name === 'custom_counter')?.metrics[0]
+          .value ?? '0'
+      )
+    ).toBe(NUMBER_OF_REQUESTS);
+  });
+
   test('Captures Counter Metrics - Router', async () => {
     expect(
       parseInt(

tests/utils.ts

@@ -2,9 +2,15 @@ import request from 'supertest';
 import express from 'express';
 import type { Express } from 'express';
 import { setOpenAPMLabels } from '../src/async-local-storage.http';
+import { metricClient } from '../src/OpenAPM';
 
 export const addRoutes = (app: Express) => {
   const router = express.Router();
+  const client = metricClient();
+  const counter = new client.Counter({
+    name: 'custom_counter',
+    help: 'no. of times operation is called'
+  });
 
   router.get('/:id', (req, res) => {
     const { id } = req.params;
@@ -14,6 +20,7 @@ export const addRoutes = (app: Express) => {
   app.use('/api/router/', router);
   app.get('/api/:id', (req, res) => {
     const { id } = req.params;
+    counter.inc();
     res.status(200).send(id);
   });