$sortKey is a special metadata field in that it requires an additional parameter to set, isSingleElementKey. In order to allow extensions to set $sortKey, we must add a special case to our parsing logic to check this value (if present) and set it.

This ticket should modify this setMetaFieldFromValue call to instead infer isSingleElementKey and call setSortKey if the metadata field is $sortKey. It looks like toBsonWithMetaDataOnly already handles this special case so no changes should be necessary there.