function addKeyFilter(oElement) {
			var sKeyFilter;

			mTypeForMetaPath ??= that.getTypes(); // Note: $metadata already read
			sKeyFilter = _Helper.getKeyFilter(oElement, that.sMetaPath, mTypeForMetaPath);
			if (sKeyFilter) {
				aKeyFilters.push(sKeyFilter);
			}
		}
Returns a filter that excludes all created entities in this cache's collection and all
entities that have been deleted on the client, but not on the server yet.

@returns {string|undefined}
  The filter or <code>undefined</code> if there is no such entity.

@private