public static Optional<String> getRoutingValue(final Model model, final ElasticSearchConverter.ToStorage toStorage) {
        if (!IndexController.INSTANCE.isEnableCustomRouting()) {
            return Optional.empty();
        }
        Optional<String> routingField = model.getElasticSearchModelExtension().getRouting();
        return routingField.map(v -> extractRoutingValue(v, toStorage));
    }
get the value of the field annotated {@link ElasticSearch.Routing}
oap-server/server-storage-plugin/storage-elasticsearch-plugin/src/main/java/org/apache/skywalking/oap/server/storage/plugin/elasticsearch/base/RoutingUtils.java