code: public MethodDescriptor<KEY> getServiceDescriptorByKey(KEY key) throws IllegalStateException {
          Preconditions.checkNotNull(key, "Key cannot be null");
          Preconditions.checkState(instance != null, "ServiceRegistry not init yet");
          return serviceDescriptors.get(key);
      }
docs: 根据标示<code>KEY</code>获取服务描述
  
  @param key
  服务的唯一标示
  @return 服务的方法描述
  @throws IllegalStateException