apollo-configservice/src/main/java/com/ctrip/framework/apollo/metaservice/service/DefaultDiscoveryService.java/DefaultDiscoveryService/getServiceInstances
class DefaultDiscoveryService:  @Override
  public List<ServiceDTO> getServiceInstances(String serviceId) {
    Application application = eurekaClient.getApplication(serviceId);
    if (application == null || CollectionUtils.isEmpty(application.getInstances())) {
      Tracer.logEvent("Apollo.Discovery.NotFound", serviceId);
      return Collections.emptyList();
    }
    return application.getInstances().stream().map(instanceInfoToServiceDTOFunc)
        .collect(Collectors.toList());
  }