code: void addLocality(final Locality locality) {
      ClientLoadCounter counter = localityLoadCounters.get(locality);
      checkState(counter == null || !counter.isActive(),
          "An active ClientLoadCounter for locality %s already exists", locality);
      if (counter == null) {
        localityLoadCounters.put(locality, new ClientLoadCounter());
      } else {
        counter.setActive(true);
      }
    }
docs: Create a {@link ClientLoadCounter} for the provided locality or make it active if already in
  this {@link XdsLoadReportStore}. This method needs to be called at locality updates only for
  newly assigned localities in balancer discovery responses.
  This method should be called in the same synchronized context that
  {@link XdsLoadBalancer#helper#getSynchronizationContext} returns.