supply metrics for cluster topology

Artemis integrates well with Prometheus for monitoring to see whether the internals of a broker work properly using the artemis-prometheus-metrics-plugin plugin.

I found that information about the presence of the broker in a broker-cluster is missing there. that information can already be found:

• in the console, tab "Status", the number of live brokers and number of backup brokers is listed
• in the JMX interfaces, the listNetworkTopology function has similar information, even with more details

suggestion is to add 2 new simple metrics:

• artemis_cluster_lives_count
• artemis_cluster_backups_count