* Fix stale admin lock metric when lock expires and is reacquired (#8857)
When a lock expired without an explicit unlock and a different client
acquired it, the old client's metric was never cleared, causing
multiple clients to appear as simultaneously holding the lock.
* Use DeleteLabelValues instead of Set(0) to remove stale metric series
Avoids cardinality explosion from accumulated stale series when
client names are dynamic.
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