This adds two metrics to the serverStatus.metrics section:

repl.waiters.replication

repl.waiters.opTime

repl.waiters.replication exposes how many threads are waiting for a replicated and/or journaled write concern to resolve. repl.waiters.opTime exposes how many threads are waiting for a local optime only.

Description of Linked Ticket

The replication waiters list can grow with the number of operations waiting for write concern. Advancing replication timestamps also requires updating all waiters in this list under a mutex. If the list is long, this can take a long time.

It would be useful to be able to see how many operations are waiting for replication in this state, which would make it easier to diagnose problems in this area.