code: public void addServiceDescriptor(KEY key, MethodDescriptor<KEY> serviceDescriptor) {
          Preconditions.checkNotNull(key, "Key cannot be null");
          Preconditions.checkState(instance != null, "ServiceRegistry not init yet");
  
          //        if (serviceDescriptors.containsKey(key)) {
          //            LOG.warn("Ignore key=" + key + " since it is already registered with "
          //                    + serviceDescriptor);
          //        }
  
          this.serviceDescriptors.putIfAbsent(key, serviceDescriptor);
      }
docs: 加入服务描述
  
  @param key
  服务的唯一标示
  @param serviceDescriptor
  服务的方法描述