Overview

Following investigation in HELP-91789, we found that there is no observability into contention on SessionCatalog::_mutex (session_catalog.h:285). This made it difficult to diagnose whether this mutex was a bottleneck during the reported incident. We should instrument it using the existing ObservableMutex infrastructure so that contention data is available for future diagnosis.

See SERVER-111639 and SERVER-111653 for prior rollouts of ObservableMutex to other critical mutexes.

Acceptance Criteria

• SessionCatalog::_mutex contention metrics appear in the lockContentionMetrics section of serverStatus:

  db.serverStatus({lockContentionMetrics: 1})

• Existing lock usage compiles and behaves correctly with the new type
• FTDC captures the metrics when enabled