public String getLabelString(String key) {
        return key + Const.EQUAL + get(key);
    }
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