1. Large arrays within documents have performance costs that are currently indirectly inferred from the behavior of wiredTiger's cache page management.

Since BSON does not encode array size (in elements), this could be a counter like "array elements visited" or (if that is deemed too expensive), it could be a counter that increments by the total byte size of each array (which is included in the BSON) when it is iterated through for any reason.

Combined with opcounters and scannedObjects (docsExamined), either of these would dramatically improve the ability to diagnose array iteration as impacting the system.