For DLQ messages that have a "doc" field, the doc._stream_meta will be set, and there is no need for a top level _stream_meta field.

If we want to fix this, it's relatively straightforward, we just need to modify the below toDeadLetterQueueMsg overload (which has a streamDoc param), to not include the top level _`stream_meta` field, and instead project doc._stream_meta.

BSONObjBuilder toDeadLetterQueueMsg(const boost::optional<std::string>& streamMetaFieldName,
                                    const StreamDocument& streamDoc,
                                    const std::string& operatorName,
                                    boost::optional<std::string> error);