Add a field that indicates whether we used classic or SBE for a query. We should backport it to 5.1. At the least we want to see how many queries are exposed to SBE.
- just logging a field if the less common case happens (e.g. "sbe")
- should we use the same field in the future for the optimizer (e.g. "sbe" (executed with sbe) / "cqf" (optimized and executed with cqf)