@Override
    public boolean combine(Metrics metrics) {
        HistogramMetrics histogramMetrics = (HistogramMetrics) metrics;
        this.dataset.append(histogramMetrics.dataset);
        return true;
    }
Data will be grouped in
<pre>
key = 0, represents [0, 100), value = count of requests in the latency range.
key = 100, represents [100, 200), value = count of requests in the latency range.
...
key = step * maxNumOfSteps, represents [step * maxNumOfSteps, MAX)
</pre>

@param step          the size of each step. A positive integer.
@param maxNumOfSteps Steps are used to group incoming value.
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/analysis/metrics/HistogramMetrics.java