code: public ProviderGroup add(ProviderInfo providerInfo) {
          if (providerInfo == null) {
              return this;
          }
          ConcurrentHashSet<ProviderInfo> tmp = new ConcurrentHashSet<ProviderInfo>(providerInfos);
          tmp.add(providerInfo); // 排重
          this.providerInfos = new ArrayList<ProviderInfo>(tmp);
          return this;
      }
docs: 增加服务列表
  
  @param providerInfo 要增加的服务分组列表
  @return 当前服务分组 provider group