private List<String> buildLabelNamesForTraffic(String metricName) {
        List<String> labelNames = new ArrayList<>();
        labelNames.add(LabelName.LAYER.getLabel());
        labelNames.add(LabelName.LIMIT.getLabel());
        labelNames.add(LabelName.SERVICE.getLabel());
        if (Objects.equals(metricName, EndpointTraffic.INDEX_NAME)) {
            labelNames.add(LabelName.KEYWORD.getLabel());
        }
        return labelNames;
    }
OAP adopt time step by start/end,
reserve step/timeout param to keep consistent with API protocol.
oap-server/server-query-plugin/promql-plugin/src/main/java/org/apache/skywalking/oap/query/promql/handler/PromQLApiHandler.java