feat: add prometheus metrics in dedicated port

package.json

@@ -59,6 +59,7 @@
     "express-winston": "^4.2.0",
     "http-errors": "^2.0.0",
     "lru-cache": "^7.13.1",
+    "prom-client": "^14.1.1",
     "rxjs": "^7.5.6",
     "winston": "^3.8.1"
   },

src/main.ts

@@ -32,6 +32,7 @@ import { Log } from './logging/Log';
 import * as middleware from './middleware';
 import { parseArgs } from './parseArgs';
 import { SidecarConfig } from './SidecarConfig';
+import Metrics_App from './util/metrics';
 
 async function main() {
 	const { config } = SidecarConfig;
@@ -88,8 +89,17 @@ async function main() {
 		host: config.EXPRESS.HOST,
 	});
 
+	// Create our App
+	const metricsApp = new Metrics_App({
+		port: 9100,
+		host: config.EXPRESS.HOST,
+	});
+
 	// Start the server
 	app.listen();
+
+	// Start the Metrics server
+	metricsApp.listen();
 }
 
 /**

src/middleware/error/httpErrorMiddleware.ts

@@ -18,6 +18,7 @@ import { ErrorRequestHandler } from 'express';
 import { HttpError } from 'http-errors';
 
 import { Log } from '../../logging/Log';
+import { httpErrorCounter } from '../../util/metrics';
 /**
  * Handle HttpError instances.
  *
@@ -47,6 +48,8 @@ export const httpErrorMiddleware: ErrorRequestHandler = (
 		stack: err.stack,
 	};
 
+	httpErrorCounter.inc();
+
 	Log.logger.error(info);
 
 	res.status(code).send(info);

src/util/metrics.ts

@@ -0,0 +1,59 @@
+import express from 'express';
+import { Application, Request, Response } from 'express';
+import client from 'prom-client';
+
+import { Log } from '../logging/Log';
+
+const { logger } = Log;
+
+const register = new client.Registry();
+
+export const httpErrorCounter = new client.Counter({
+	name: 'sas_http_errors',
+	help: 'Number of HTTP Errors',
+});
+
+register.registerMetric(httpErrorCounter);
+
+client.collectDefaultMetrics({ register, prefix: 'sas_' });
+
+interface IAppConfiguration {
+	port: number;
+	host: string;
+}
+
+export default class Metrics_App {
+	private app: Application;
+	private readonly port: number;
+	private readonly host: string;
+
+	/**
+	 * @param appConfig configuration for app.
+	 */
+	constructor({ host }: IAppConfiguration) {
+		this.app = express();
+		this.port = 9100;
+		this.host = host;
+
+		this.metricsEndpoint();
+	}
+
+	listen(): void {
+		this.app.listen(this.port, this.host, () => {
+			logger.info(
+				`Metrics Server started at http://${this.host}:${this.port}/`
+			);
+		});
+	}
+
+	/**
+	 * Mount the metrics endpoint.
+	 */
+	private metricsEndpoint() {
+		// Set up the metrics endpoint
+		this.app.get('/metrics', async (_req: Request, res: Response) => {
+			res.set('Content-Type', register.contentType);
+			return res.send(await register.metrics());
+		});
+	}
+}