Create a shard collection with key that isn't _id. $merge writes will send things to the DLQ with:
{
_id: ObjectId("651d8464f0b07ba2ac0ce8a3"),
_stream_meta:

,
errInfo: {
reason: 'Failed to process an input document in the current batch in MergeOperator with error: Failed to target upsert by query :: could not extract exact shard key: generic server error'
}
}