Append the backup cursors user arguments to the metadata document

> primary.getDB("admin").aggregate([{$backupCursor: {incrementalBackup: true, thisBackupName: "bar", srcBackupName: "foo"}}]);

 {

    "metadata" : {

        "backupId" : UUID("83f0327d-5194-4e14-b3bb-1c676450794d"),

        "dbpath" : "/data/db/job0/mongorunner/backup_cursor_format-0",

        "oplogStart" : {

            "ts" : Timestamp(1582568119, 1),

            "t" : NumberLong(-1)

        },

        "oplogEnd" : {

            "ts" : Timestamp(1582568162, 1),

            "t" : NumberLong(1)

        },

        "checkpointTimestamp" : Timestamp(1582568162, 1),

        "disableIncrementalBackup" : false,

        "incrementalBackup" : true,

        "blockSize" : 16,

        "thisBackupName" : "bar",

        "srcBackupName" : "foo"

    }

 }