Metrics per core: report leader true/false and replica state

The core level metrics hold some interesting information, but I don't see information pertaining to the SolrCloud status of the core.  In particular, I'd like to see the leader status here, and also the replica state.  The use-case I have in mind is enabling the Prometheus Exporter to get the doc count (and maybe other basics) of only the leader replicas, thereby counting unique documents instead of a fully replicated figure.  This is an approximation to doing a match-all-docs query on all collections, but is a more sound approach when one has orders of magnitude more collections than nodes.

Final change:  add "isLeader" and "replicaState" metrics per core.