numEmptyBatches is a new field under metrics.repl.network.getmores. This number reports the number of empty oplog batches a secondary node receives from its sync source. A secondary will get an empty oplog batch only when it is fully synced (i.e. the oplog getmore operation does not initial return data) and if:
1. The getmore operation also times out waiting for more data (oplog entries).
2. The sync source's majority commit point has advanced since the last batch sent to the secondary node.