apollo-adminservice/src/main/java/com/ctrip/framework/apollo/adminservice/controller/ClusterController.java/ClusterController/get
class ClusterController:  @GetMapping("/apps/{appId}/clusters/{clusterName:.+}")
  public ClusterDTO get(@PathVariable("appId") String appId,
                        @PathVariable("clusterName") String clusterName) {
    Cluster cluster = clusterService.findOne(appId, clusterName);
    if (cluster == null) {
      throw new NotFoundException("cluster not found for name " + clusterName);
    }
    return BeanUtils.transform(ClusterDTO.class, cluster);
  }