docs: synchronized void onComplete(@javax.annotation.Nullable Throwable error) {
      if (isCompleted()) {
        return;
      }
      currentConfigObserver = null;
      // TODO(songya): add Runnable
      Status status;
      if (error == null) {
        status = Status.OK;
      } else if (error instanceof StatusRuntimeException) {
        status = ((StatusRuntimeException) error).getStatus();
      } else {
        status = Status.UNKNOWN;
      }
      terminateStatus = status;
    }