code: public Collection<MethodDescriptor<KEY>> getAllServiceDescriptors()
              throws IllegalStateException {
          Preconditions.checkNotNull(serviceDescriptors, "serviceDescriptors cannot be null");
          return serviceDescriptors.values();
      }
docs: 获取所有服务描述
  
  @return 所有注册的服务的方法描述
  @throws IllegalStateException