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).
OR
2. The sync source's majority commit point has advanced since the last batch sent to the secondary node.