As a part of SPM-3391 we added optimization time to the top-level of query stats. However, it is useful to know the optimization time used by CBR vs by the multi-planner.

For example, we could have a case where a query tries to use CBR, samples some documents, but falls back to multi-planning. Query stats would output CBR metrics but the optimization time would reflect the entire optimization time without giving us a sense of how much CBR succeeded. 

We already collect optimization times grouped by engine used (classic, bonsai) and the idea is that we should add another grouping for CBR: https://github.com/10gen/mongo/blob/484400ac9be5459b64ac7367b248d24b1fc645f0/src/mongo/db/query/query_stats/supplemental_metrics_stats.cpp#L67

note: these supplemental stats are only collected when this knob is enabled