dispatchMetadataEvent_(region, eventName) {
    const data = new Map()
        .set('startTime', region.startTime)
        .set('endTime', region.endTime)
        .set('metadataType', region.schemeIdUri)
        .set('payload', region.payload);
    this.dispatchEvent(shaka.Player.makeEvent_(eventName, data));
  }
Construct and fire metadata event of given name

@param {shaka.extern.MetadataTimelineRegionInfo} region
@param {shaka.util.FakeEvent.EventName<string>} eventName
@private