private List<Service> combineServices(List<Service> services) {
        if (CollectionUtils.isEmpty(services)) {
            return Collections.emptyList();
        }
        return new ArrayList<>(services.stream()
                                       .collect(Collectors.toMap(Service::getName, service -> service,
                                                                 (s1, s2) -> {
                                                                     s1.getLayers().addAll(s2.getLayers());
                                                                     return s1;
                                                                 }
                                       )).values());
    }
@return all services, key is service id, value is services with different layers.
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/query/MetadataQueryService.java