Explain command does not report executionStats for the merger part of a split pipeline

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      I've noticed that for sharded collections, we report executionStats under the shards field for each sub-pipeline that gets executed on the individual shards. But we don't have any information about what is happening on mongos during the $mergeCursors or the stages after the $mergeCursors. For example, a $group or a $sort could spend a significant amount of time after the $mergeCursors and we provide no information about that in the explain. There could also be a network latency between the mongos and mongod communication that would be easy to identify if we report executionStats on mongos.

              Assignee:
              [DO NOT USE] Backlog - Query Execution
              Reporter:
              Arun Banala
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                Created:
                Updated: