apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/controller/InstanceController.java/InstanceController/getInstanceCountByNamespace
class InstanceController:    @GetMapping("/envs/{env}/instances/by-namespace/count")
    public ResponseEntity<Number> getInstanceCountByNamespace(@PathVariable String env, @RequestParam String appId,
                                                              @RequestParam String clusterName,
                                                              @RequestParam String namespaceName) {

        int count = instanceService.getInstanceCountByNamespace(appId, Env.valueOf(env), clusterName, namespaceName);
        return ResponseEntity.ok(new Number(count));
    }