Today, the oplog tailer uses tryExtractNoopRepair() to deserialize repairReplicatedMetadata oplog entries. The motivation for using this function instead of the IDL-generated RepairReplicatedMetadataO2::parse() is parsing in the oplog tailer is performance-sensitive. However, we only call tryExtractNoopRepair() after determining an entry is a no-op, has an o2 field, and that o2 field is a repair metadata command:

            case 'n':
                if (f.o2.type() == BSONType::object && isRepairReplicatedMetadataO2(f.o2.Obj())) {
                    return FastDecision::kNoopRepair;

Thus the cost of IDL-parsing is only paid for valid or malformed repairReplicatedCommand entries.

Additionally, we do not use the IDL-generated serialization API when writing the no-op entry:

            BSON("msg" << "Repairing collection's replicated metadata with diffs"),
            BSON("type" << "repairReplicatedMetadata" << "uuid" << collection.uuid() << "m"
                        << metadataObj),

We can simplify this code by using the pre-existing IDL serialization/deserialization API. For example, in _writeNoopOplogEntry(), we can replace the hand-built o2 with 

const auto o2 = RepairReplicatedMetadataO2{...};

This change has the added benefit of unifying the parsing performed on the standby in SERVER-137710, thus preventing the primary and standby from accidentally parsing the same entries differently.