add connected client count metric

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

@@ -56,6 +56,7 @@ module.exports = Router =
 			client.emit("connectionAccepted")
 
 			metrics.inc('socket-io.connection')
+			metrics.gauge('socket-io.clients', io.sockets.clients()?.length)
 
 			logger.log session: session, client_id: client.id, "client connected"
 
@@ -77,6 +78,7 @@ module.exports = Router =
 
 			client.on "disconnect", () ->
 				metrics.inc('socket-io.disconnect')
+				metrics.gauge('socket-io.clients', io.sockets.clients()?.length)
 				WebsocketController.leaveProject io, client, (err) ->
 					if err?
 						Router._handleError null, err, client, "leaveProject"