We've started using no-op oplog entries as ways to convey internal information throughout the system - for example in change streams and as part of retriable writes. This usage is likely to increase with future transaction work. No-op oplog entries, however, do not currently have an OpObserver. We should add an easy way for secondaries (and maybe also primaries?) to listen for and take action on any no-op oplog entries, potentially by working them into the existing OpObserver system or potentially via a new mechanism