The DrillSideways search methods will always populate a FacetsCollector for the "drill down" dimensions in addition to the "drill sideways" dimensions. For most cases, this makes sense, but it would be nice if users had a way to opt-out of this collection. It's possible a user may not care to do any faceting on "drill down" dims, or may have custom needs for facet collecting on the "drill downs." For the latter case, the user might want to provide a Collector/CollectorManager that does facet collecting with some custom logic (e.g., behind a MultiCollector/MultiCollectorManager), in which case the population of an additional FacetsCollector in DrillSideways is wasteful.
The DrillSidewaysScorer already supports a null drillDownCollector gracefully, so this change should mostly just involve creating a protected method in DrillSideways for the purpose of creating a "drill down" FacetsCollector that users can override by providing null.