Rabbitmq Queue Length Metric required

Created by: hughsaunders

"For the RabbitMQ queue length, the scenario we want to detect is a queue growing due to a failed consumer or consumers that can’t keep up with load."

"Support confirmed (via @bjoern) that monitoring and alerting on the following single metric would be sufficient: ((Total message count across all queues) – (messages in notifications queue))"

Further details: https://trello.com/c/widYAV5F/35-add-new-monitor-rabbitmq-queue-length