Running coll.aggregate([\{$planCacheStats: {}}]) on a time-series collection will fail with
"$planCacheStats is only valid as the first stage in a pipeline".
 
In case of time-series pipelines, $_internalUnpackBucket is the first stage. We need special handling for stages with constraints on pipeline position like the following for $indexStats. https://github.com/10gen/mongo/blob/master/src/mongo/db/views/resolved_view.cpp/#L137-L161