Expose number of unreachable cluster peers via Prometheus

[gomoripeti]

Proposed Changes

Unreachable peers is a subset of DB cluster nodes that are not connected to the current node via Erlang distribution for any reason.

As discussed in #9376
Closes #2508 (to the extent agreed to in #9376)

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[michaelklishin]

Start a three node cluster with bazel run start-cluster, shut down two nodes out of three, observe

# TYPE rabbitmq_unreachable_cluster_peers_count gauge
# HELP rabbitmq_unreachable_cluster_peers_count Number of peers in the cluster the current node cannot reach.
rabbitmq_unreachable_cluster_peers_count 2