@Override
    public String toString() {
        Iterator<Map.Entry<String, String>> i = entrySet().iterator();
        if (!i.hasNext()) {
            return "{}";
        }
        StringBuilder sb = new StringBuilder();
        sb.append('{');
        for (; ; ) {
            Map.Entry<String, String> e = i.next();
            String key = e.getKey();
            String value = e.getValue();
            sb.append(key);
            sb.append('=');
            sb.append(value);
            if (!i.hasNext()) {
                return sb.append('}').toString();
            }
            sb.append(',');
        }
    }
Parse the labels String to a map.

@param labels the String format is {key1=value1,key2=value1}, if not match the format will use `_` as the key and
              the `labels string` as the value.
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/analysis/metrics/DataLabel.java