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

    debugBuild.DEBUG_BUILD && utils.logger.log('[Measurements] Adding CLS');
    _measurements['cls'] = { value: metric.value, unit: '' };
    _clsEntry = entry ;
  }, true);
}
Starts tracking the Cumulative Layout Shift on the current page.