Adds QueueUsage to prometheus metrics (same as /api/stat/app) (#401)
Adds QueueUsage to prometheus metrics (same as /api/stat/app) (#401)

Exposes gorush_queue_usage metric to be used in prometheus so monitor internal queues

Useful to make sure we are coping well with traffic
appleboy/gorush,appleboy/gorush,appleboy/gorush,appleboy/gorush