Please see the preceding work here.

Investigate whether replacing ObservableMutex inline acquisition stats with direct OTel counters introduces noticeable performance overhead on the hot path.

If the overhead is acceptable, proceed with replacing inline stats with direct counters. Otherwise, look into implementing a specialized OTel counter wrapper for this performance-critical case and re-validate performance before moving forward.

Finally, remove `installObservableMutexMetrics` from the metrics installer.