function addClsInstrumentationHandler(
  callback,
  stopOnCallback = false,
) {
  return addMetricObserver('cls', callback, instrumentCls, _previousCls, stopOnCallback);
}
Add a callback that will be triggered when a CLS metric is available.
Returns a cleanup callback which can be called to remove the instrumentation handler.

Pass `stopOnCallback = true` to stop listening for CLS when the cleanup callback is called.
This will lead to the CLS being finalized and frozen.