add metric for RoomEvents listeners

services/real-time/app/coffee/RoomManager.coffee

@@ -1,4 +1,5 @@
 logger = require 'logger-sharelatex'
+metrics = require "metrics-sharelatex"
 {EventEmitter} = require 'events'
 
 IdMap = new Map() # keep track of whether ids are from projects or docs
@@ -55,6 +56,8 @@ module.exports = RoomManager =
                 callback(err)
             RoomEvents.emit "#{entity}-active", id
             IdMap.set(id, entity)
+            # keep track of the number of listeners
+            metrics.gauge "room-listeners", RoomEvents.eventNames().length
         else
             logger.log {client: client.id, entity, id, beforeCount}, "client joined existing room"
             client.join id
@@ -72,6 +75,7 @@ module.exports = RoomManager =
             logger.log {entity, id}, "room is now empty"
             RoomEvents.emit "#{entity}-empty", id
             IdMap.delete(id)
+            metrics.gauge "room-listeners", RoomEvents.eventNames().length
 
     # internal functions below, these access socket.io rooms data directly and
     # will need updating for socket.io v2

services/real-time/test/unit/coffee/RoomManagerTests.coffee

@@ -13,6 +13,7 @@ describe 'RoomManager', ->
 		@RoomManager = SandboxedModule.require modulePath, requires:
 			"settings-sharelatex": @settings = {}
 			"logger-sharelatex": @logger = { log: sinon.stub(), error: sinon.stub() }
+			"metrics-sharelatex": @metrics = { gauge: sinon.stub() }
 		@RoomManager._clientsInRoom = sinon.stub()
 		@RoomEvents = @RoomManager.eventSource()
 		sinon.spy(@RoomEvents, 'emit')