docs: public Map<String, OperationResult<String>> sendHeartBeat(Map<String, ServiceInstanceHeartbeat> heartbeatMap){
          String body = _serialize(heartbeatMap);
          HttpResponse result = invoker.invoke("/service/heartbeat", body,
                  HttpMethod.PUT);
  
          if (result.getHttpCode() != HTTP_OK) {
             throw new ServiceException(ErrorCode.REMOTE_DIRECTORY_SERVER_ERROR,
                      "HTTP Code is not OK, code=%s", result.getHttpCode());
          }
  
          Map<String, OperationResult<String>> operateResult = _deserialize(
                  result.getRetBody(), new TypeReference<Map<String, OperationResult<String>>>(){});
          return operateResult;
  
      }