A simple query with an index and $meta fails.
Right now the query from the "Steps To Reproduce" section hits an invariant failure: reqs.has(kReturnKey) || !ixn->addKeyMetadata .
I tried fixing this invariant by calling PlanStageReqs::set() at the top level to set kReturnKey, and then I re-ran the query from the "Steps To Reproduce" section. When I did this, I got the following error message: "Expression is not supported in SBE: $meta".
So it looks like we need to update "sbe_stage_builder_expression.cpp" so that it can handle ExpressionMeta with _metaType == MetaType::kIndexKey .