function _trackLCP() {
  return instrument.addLcpInstrumentationHandler(({ metric }) => {
    const entry = metric.entries[metric.entries.length - 1];
    if (!entry) {
      return;
    }

    debugBuild.DEBUG_BUILD && utils.logger.log('[Measurements] Adding LCP');
    _measurements['lcp'] = { value: metric.value, unit: 'millisecond' };
    _lcpEntry = entry ;
  }, true);
}
Starts tracking the Largest Contentful Paint on the current page.