docs: 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);
      }
    }