add metric for pendingUpdates queue

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

@@ -72,6 +72,9 @@ module.exports = DocumentUpdaterManager =
 			error.updateSize = updateSize
 			return callback(error)
 
+		# record metric for each update added to queue
+		metrics.summary 'redis.pendingUpdates', updateSize, {status: 'push'}
+
 		doc_key = "#{project_id}:#{doc_id}"
 		# Push onto pendingUpdates for doc_id first, because once the doc updater
 		# gets an entry on pending-updates-list, it starts processing.

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

@@ -25,6 +25,7 @@ describe 'DocumentUpdaterManager', ->
 				'request': @request = {}
 				'redis-sharelatex' : createClient: () => @rclient
 				'metrics-sharelatex': @Metrics =
+					summary: sinon.stub()
 					Timer: class Timer
 						done: () ->
 			globals: