GroupProcessorBase already creates and uses a tracker to account for its permanent state memory usage and derives from it a SimpleMemoryUsageTracker for each accumulated field. This per-field memory tracker will be passed when the accumulator's initialiser expression is evaluated and when its argument expression is evaluated. The group-by key expression is evaluated separately and needs its own memory tracker which will be derived from the initial memory tracker, similar to how per-accumulator trackers are created, using _memoryTracker["_id"].

The flags gFeatureFlagExpressionMemoryTracking and gFeatureFlagMemoryTracking must be checked before passing the tracker.