getExtent(extent) {
    return this.featuresRtree_.getExtent(extent);
  }
Get the extent of the features currently in the source.

This method is not available when the source is configured with
`useSpatialIndex` set to `false`.
@param {import("../extent.js").Extent} [extent] Destination extent. If provided, no new extent
    will be created. Instead, that extent's coordinates will be overwritten.
@return {import("../extent.js").Extent} Extent.
@api