code: public static Cluster getCluster(ConsumerBootstrap consumerBootstrap) {
          try {
              ConsumerConfig consumerConfig = consumerBootstrap.getConsumerConfig();
              ExtensionClass<Cluster> ext = ExtensionLoaderFactory.getExtensionLoader(Cluster.class)
                  .getExtensionClass(consumerConfig.getCluster());
              if (ext == null) {
                  throw ExceptionUtils.buildRuntime("consumer.cluster",
                      consumerConfig.getCluster(), "Unsupported cluster of client!");
              }
              return ext.getExtInstance(new Class[] { ConsumerBootstrap.class },
                  new Object[] { consumerBootstrap });
          } catch (SofaRpcRuntimeException e) {
              throw e;
          } catch (Throwable e) {
              throw new SofaRpcRuntimeException(e.getMessage(), e);
          }
      }
docs: 构造Cluster对象
  
  @param consumerBootstrap 客户端配置
  @return Cluster对象