Add counters for Rebalance status

Please add a new metric family to track the number of rebalance events, matching the same event names as used in the System Event Log.

Proposal is a new metric named cm_rebalance_count, with a single label (event), with values initiated, interrupted, completed, failed.

The count of initiated would increment for every Rebalance that starts, and one of interrupted, completed, or failed would increment for every one that finishes.