From damian.wasilewicz@mongodb.com,

Some additional metrics that I think would be useful/that would have been useful during the perf investigation are metrics around the _metadata mutex. I see that for other hotly contended mutexes (i.e, ReplicationCoordinatorImpl, OplogApplier, etc) we have insight into acquisition totals, rates, and waitCycles. I feel like this could be useful here as well.