apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/api/AdminServiceAPI.java/AdminServiceAPI/getInstanceCountByNamespace
class AdminServiceAPI:    public int getInstanceCountByNamespace(String appId, Env env, String clusterName, String namespaceName) {
      Integer
          count =
          restTemplate.get(env,
              "/instances/by-namespace/count?appId={appId}&clusterName={clusterName}&namespaceName={namespaceName}",
              Integer.class, appId, clusterName, namespaceName);
      if (count == null) {
        return 0;
      }
      return count;
    }