Under PM-1945 the simplest approach to allow for $_internalUnpackBucket to work in a sharded environment was to push $_internalUnpackBucket onto the shards and send the unpacked measurements to mongos. This ticket encapsulates work that could be a good optimization to reduce network traffic between mongod and mongos. We should explore when the tipping point would be given metadata size, the number of included/excluded projections when it would be cheaper to send buckets between mongos and mongod and unpack on the merging node.