If replication is enabled, the $backupCursor document source should additionally return a metadata document with three fields:

{
  checkpointTImestamp: <BSONTimestamp>,
  oplogStart: <OpTime>,
  oplogEnd: <OpTime>
}

getLastStableCheckpointTimestamp should be used to acquire the checkpointTimestamp. Additionally, the design document dictates how to obtain the oplog start/end optimes.

Note that during initial sync, the checkpointTimestamp may not exist. A backup in that state is of questionable value, but this project will not prohibit an application from taking one.