private synchronized Long gauge() {
        if (currentValue == previousValue) {
            return null;
        } else {
            previousValue = currentValue;
            return currentValue;
        }
    }
Gauge metric for event time lag.
inlong-sort/sort-formats/format-rowdata/format-rowdata-base/src/main/java/org/apache/inlong/sort/formats/metrics/gauge/EventTimeDelayGauge.java