Provide metrics for number of total active, priority and replication rpc handlers

Currently hbase provides a metric for number of total active rpc handlers which is a sum of the following:

• number of active general rpc handlers
• number of active priority rpc handlers
• number of active replication rpc handlers

I think we can have 3 different metrics corresponding to the above mentioned handlers which will allow us to see detailed information about number of active handlers running for a particular type of handler.

We can have following new metrics:

• numActiveGeneralHandler
• numActivePriorityHandler
• numActiveReplicationHandler