apollo-adminservice/src/main/java/com/ctrip/framework/apollo/adminservice/controller/InstanceConfigController.java/InstanceConfigController/getInstancesCountByNamespace
class InstanceConfigController:  @GetMapping("/by-namespace/count")
  public long getInstancesCountByNamespace(@RequestParam("appId") String appId,
                                          @RequestParam("clusterName") String clusterName,
                                          @RequestParam("namespaceName") String namespaceName) {
    Page<Instance> instances = instanceService.findInstancesByNamespace(appId, clusterName,
        namespaceName, PageRequest.of(0, 1));
    return instances.getTotalElements();
  }