The counter for the "sent" metric is incremented after scheduling the remote command request for the mirrored read, rather than after the command is actually sent. In the event of a timeout getting a connection or a network error sending the command, this metric would have been incremented incorrectly. We should instead only increment this metric after the request has successfully been sent to the secondaries.

https://github.com/10gen/mongo/blob/e1cc00b1d81b4367f2b888556b4bad00ddedee70/src/mongo/db/mirror_maestro.cpp#L516