Add diagnosability and supportability features for retryable writes

This ticket is a placeholder to figure out between ourselves and with TSE what supportability and diagnosability enhancements we might need to include for retryable writes.

Things that come to mind include:

• FTDC metrics for number of active sessions
• Metrics for the CPU (and if possible I/O) overhead of maintaining the retryability metadata
• Metrics for number of writes which have been retried
• If we end up with a cache, metrics for hit-rate