static async computeObservedMetric(data, context) {
    const speedline = await Speedline.request(data.trace, context);
    const timing = Math.round(speedline.speedIndex);
    const timestamp = (timing + speedline.beginning) * 1000;
    return Promise.resolve({timing, timestamp});
  }
@param {LH.Artifacts.NavigationMetricComputationData} data
@param {LH.Artifacts.ComputedContext} context
@return {Promise<LH.Artifacts.Metric>}