Add Metrics for Rebalance Progress

Add metrics to allow exposing the rebalance progress via the Stats and Prometheus endpoints.

Initial suggestion is to keep it simple:

• A single new metric family: cm_rebalance_progress (Type gauge, Units ratio)
• A label for the stage/service: stage (data, index, query, etc.)

At each scrape, return one entry per service, with the stage label and the progress of that stage/service. This metric will only be up-to-date on the current orchestrator node.