After SERVER-92457, we will have the ability to fully push down $groups to shards (no residual $group on merging pipeline) in cases where the _id field is a superset of the shard key. We initially disabled this optimization for cases where the $exchange optimization can kick in, in order to minimize the effect of that ticket, but we should be able to do both optimizations together, since such a $group should "preserve" data that is partitioned by chunks owned by a given shard (i.e. no overlap between groups produced by different shards).