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/MapController.ts

@@ -19,7 +19,7 @@ export class MapController {
     // Returns a map mapping map name to file name of the map
     getStartMap() {
         this.App.get("/start-map", (req: Request, res: Response) => {
-            return res.status(OK).send({
+            res.status(OK).send({
                 mapUrlStart: req.headers.host + "/map/files" + URL_ROOM_STARTED,
                 startInstance: "global"
             });