In the future it will be more difficult to determine which queries use SBE and which use classic. Let's add a counter in query stats for which engine was used. It can be in the same place the plan shape counters are counted:

https://github.com/10gen/mongo/blob/aeb839c4b21a0d5b845cebf31258582946ee5d57/src/mongo/db/query/get_executor_deferred_engine_choice_lowering.cpp#L69-L82