* master: delete replica_placement_mismatch labels when volumes leave topology
Fixes#10804. Setting the gauge to 0 left stale Prometheus time series
that grew unbounded with volume churn; remove the label set on unregister
instead.
* master: delete replica_placement_mismatch only after last placement leaves
Unconditional DeleteLabelValues on UnRegisterVolumeLayout dropped the series
while other data nodes still held the volume, hiding under-replication until
the next collect cycle. Delete only when Lookup is empty, and cover the
two-copy case in a regression test.
