Add message rate metrics to RabbitMQ check.

This is useful when you have a queue that's consumed very quickly. In
this case the Agent will always/nearly always report 0 for queue sizes.

New metrics:
    - `rabbitmq.queue.messages.rate`
    - `rabbitmq.queue.messages_ready.rate`
    - `rabbitmq.queue.messages_unacknowledged.rate`