@Override
    public final boolean combine(Metrics metrics) {
        AvgFunction longAvgMetrics = (AvgFunction) metrics;
        this.summation += longAvgMetrics.summation;
        this.count += longAvgMetrics.count;
        return true;
    }
Service ID is required for sort query.
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/analysis/meter/function/avg/AvgFunction.java