See https://github.com/mongodb/specifications/blob/e9b4c96f3ff9b639c61feb7ed149ed14b59aabb6/source/command-logging-and-monitoring/command-logging-and-monitoring.rst#common-fields

The lack of support for operationId makes it difficult to correlate events and log messages for 

• connection check out started and connection check out completed/failed
• bulk writes
• cursor iteration 

So there is no way to reliably determine, for example, how long an operation spent in a connection pool's wait queue.