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/App.ts

@@ -6,6 +6,7 @@ import {Application, Request, Response} from 'express';
 import bodyParser = require('body-parser');
 import * as http from "http";
 import {MapController} from "./Controller/MapController";
+import {PrometheusController} from "./Controller/PrometheusController";
 
 class App {
     public app: Application;
@@ -13,6 +14,7 @@ class App {
     public ioSocketController: IoSocketController;
     public authenticateController: AuthenticateController;
     public mapController: MapController;
+    public prometheusController: PrometheusController;
 
     constructor() {
         this.app = express();
@@ -29,6 +31,7 @@ class App {
         this.ioSocketController = new IoSocketController(this.server);
         this.authenticateController = new AuthenticateController(this.app);
         this.mapController = new MapController(this.app);
+        this.prometheusController = new PrometheusController(this.app, this.ioSocketController);
     }
 
     // TODO add session user
@@ -49,4 +52,4 @@ class App {
     }
 }
 
-export default new App().server;
+export default new App().server;