Adding Prometheus metrics

This commit adds a '/metrics' endpoint in the API that can be exploited by Prometheus.

This endpoint returns:

- the number of connected sockets
- the number of users per room
- common NodeJS and system metrics

WARNING: this endpoint is public right now and should be protected

back/src/Controller/PrometheusController.ts

@@ -0,0 +1,20 @@
+import {Application, Request, Response} from "express";
+import {IoSocketController} from "_Controller/IoSocketController";
+const register = require('prom-client').register;
+const collectDefaultMetrics = require('prom-client').collectDefaultMetrics;
+
+export class PrometheusController {
+    constructor(private App: Application, private ioSocketController: IoSocketController) {
+        collectDefaultMetrics({
+            timeout: 10000,
+            gcDurationBuckets: [0.001, 0.01, 0.1, 1, 2, 5], // These are the default buckets.
+        });
+
+        this.App.get("/metrics", this.metrics.bind(this));
+    }
+
+    private metrics(req: Request, res: Response): void {
+        res.set('Content-Type', register.contentType);
+        res.end(register.metrics());
+    }
+}