public Map<String, Set<String>> buildLabelIndex() {
        Map<String, Set<String>> labelIndex = new HashMap<>();
        for (String key : data.keySet()) {
            if (key.startsWith(Const.LEFT_BRACE)) {
                String labels = key.substring(1, key.length() - 1);
                if (StringUtil.isNotEmpty(labels)) {
                    String[] labelsArr = labels.split(Const.COMMA);
                    for (String label : labelsArr) {
                        labelIndex.computeIfAbsent(label, keys -> new HashSet<>()).add(key);
                    }
                }
            } else {
                labelIndex.computeIfAbsent(GENERAL_LABEL_NAME + Const.EQUAL + key, keys -> new HashSet<>()).add(key);
            }
        }
        return labelIndex;
    }
@return the sum of all values.
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/analysis/metrics/DataTable.java