Export quorum queue member metrics

[zaeemarshad]

WHAT

Exposes members_len and online_len metric for quorum queues. The members_len shows the number of Raft members the queue was started with while online_len shows the current number of raft members the queue has. Under normal scenario, these two numbers are same. However, when a node is replaced, the raft membership is actually reduced and has to be manually increased. These two metrics can be used to alert for such a situation where the online nodes length is less than the expected members length.

Proof

# HELP rabbitmq_queue_online_nodes_len Number of online members nodes for the queue
# TYPE rabbitmq_queue_online_nodes_len gauge
rabbitmq_queue_online_nodes_len{cluster="rabbit@rabbit-1",durable="true",policy="",queue="qq-test-2",self="0",vhost="/"} 2

# HELP rabbitmq_queue_member_nodes_len Number of quorum queue member nodes for the queue
# TYPE rabbitmq_queue_member_nodes_len gauge
rabbitmq_queue_member_nodes_len{cluster="rabbit@rabbit-1",durable="true",policy="",queue="qq-test-2",self="0",vhost="/"} 3

[kbudde]

@zaeemarshad I decided to stop accepting new features. Your change looks quite simple.
If you can solve the conflicts, I would put it into a release.

[zaeemarshad]

@zaeemarshad I decided to stop accepting new features. Your change looks quite simple. If you can solve the conflicts, I would put it into a release.

@kbudde Thanks - I have resolved the conflicts.