apollo-configservice/src/main/java/com/ctrip/framework/apollo/metaservice/service/SpringCloudInnerDiscoveryService.java/SpringCloudInnerDiscoveryService/getServiceInstances
class SpringCloudInnerDiscoveryService:    @Override
    public List<ServiceDTO> getServiceInstances(String serviceId) {
        List<ServiceInstance> instances = discoveryClient.getInstances(serviceId);
        List<ServiceDTO> serviceDTOList = Lists.newLinkedList();
        if (!CollectionUtils.isEmpty(instances)) {
            instances.forEach(instance -> {
                ServiceDTO serviceDTO = this.toServiceDTO(instance, serviceId);
                serviceDTOList.add(serviceDTO);
            });
        }
        return serviceDTOList;
    }