$vectorSearch currently has supplemental metrics, limit and numCandidatesLimitRatio, that get exposed via $queryStats. These are specific to $vectorSearch and are not automatically collected like the rest of query stats. These supplemental metrics are initialized here.  These are set at parse-time. When a $queryStats runs, the $vectorSearch supplemental stats are added to the resulting object if they have been set. An example of how this gets exposed to the user via $queryStats can be found here.

This ticket may be dependent on a larger effort support supplemental metrics for extension stages. Update the dependencies as necessary.