added comment

Author: martijnvg

x-pack/plugin/downsample/src/main/java/org/elasticsearch/xpack/downsample/MetricFieldProducer.java

@@ -166,6 +166,8 @@ public void collect(SortedNumericDoubleValues docValues, IntArrayList docIdBuffe
                         case min -> min = Math.min(value, min);
                         case max -> max = Math.max(value, max);
                         case sum -> sum.add(value);
+                        // This is the reason why we can't use GaugeMetricFieldProducer
+                        // For downsampled indices aggregate metric double's value count field needs to be summed.
                         case value_count -> count += Math.round(value);
                     }
                 }