code: public void mergeHistograms(MetricInfo metricInfo, String meta, Map<Integer, MetricSnapshot> data,
                                  Map<String, Integer> metaCounters, Map<String, Map<Integer, Histogram>> histograms) {
          Map<Integer, MetricSnapshot> existing = metricInfo.get_metrics().get(meta);
          if (existing == null) {
              metricInfo.put_to_metrics(meta, data);
              Map<Integer, Histogram> histogramMap = new HashMap<>();
              for (Map.Entry<Integer, MetricSnapshot> dataEntry : data.entrySet()) {
                  Histogram histogram = MetricUtils.metricSnapshot2Histogram(dataEntry.getValue());
                  histogramMap.put(dataEntry.getKey(), histogram);
              }
              histograms.put(meta, histogramMap);
          } else {
              for (Map.Entry<Integer, MetricSnapshot> dataEntry : data.entrySet()) {
                  Integer win = dataEntry.getKey();
                  MetricSnapshot snapshot = dataEntry.getValue();
                  MetricSnapshot old = existing.get(win);
                  if (old == null) {
                      existing.put(win, snapshot);
                      histograms.get(meta).put(win, MetricUtils.metricSnapshot2Histogram(snapshot));
                  } else {
                      if (snapshot.get_ts() >= old.get_ts()) {
                          old.set_ts(snapshot.get_ts());
                          Histogram histogram = histograms.get(meta).get(win);
                          Snapshot updateSnapshot = histogram.getSnapshot();
                          if (updateSnapshot instanceof JAverageSnapshot) {
                              sumMetricSnapshot(((JAverageSnapshot) updateSnapshot).getMetricSnapshot(), snapshot);
                          } else {
                              // update points
                              MetricUtils.updateHistogramPoints(histogram, snapshot.get_points(), snapshot.get_pointSize());
                          }
                      }
                  }
              }
          }
          updateMetricCounters(meta, metaCounters);
      }
docs: histograms are sampled, but we just update points