Previously the reshard orchestrator config had a None metrics_callback,
meaning no Prometheus metrics were emitted during reshard operations.
This commit implements the metrics callback to update:
- miroir_reshard_in_progress: gauge set to 1 during active resharding, 0 when idle/complete/failed
- miroir_reshard_phase: gauge tracking current phase (0=idle, 1=shadow, 2=dual_write, 3=backfill, 4=verify, 5=swapped, 6=cleanup, 7=complete, 8=failed)
- miroir_reshard_documents_backfilled_total: counter incremented with document counts during backfill and later phases
The callback uses the public Metrics API methods (set_reshard_in_progress,
set_reshard_phase, inc_reshard_documents_backfilled) and correctly maps
ReshardPhase enum variants to their corresponding phase numbers.
Closes: bf-4wza
a7d501dc77