otlp: Document regular and exponential histogram conversions

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

storage/remote/otlptranslator/prometheusremotewrite/helper.go

@@ -220,6 +220,13 @@ func isValidAggregationTemporality(metric pmetric.Metric) bool {
 	return false
 }
 
+// addHistogramDataPoints adds OTel histogram data points to the corresponding Prometheus time series
+// as classical histogram samples.
+//
+// Note that we can't convert to native histograms, since these have exponential buckets and don't line up
+// with the user defined bucket boundaries of non-exponential OTel histograms.
+// However, work is under way to resolve this shortcoming through a feature called native histograms custom buckets:
+// https://github.com/prometheus/prometheus/issues/13485.
 func (c *PrometheusConverter) addHistogramDataPoints(dataPoints pmetric.HistogramDataPointSlice,
 	resource pcommon.Resource, settings Settings, baseName string) {
 	for x := 0; x < dataPoints.Len(); x++ {

storage/remote/otlptranslator/prometheusremotewrite/histograms.go

@@ -30,6 +30,8 @@ import (
 
 const defaultZeroThreshold = 1e-128
 
+// addExponentialHistogramDataPoints adds OTel exponential histogram data points to the corresponding time series
+// as native histogram samples.
 func (c *PrometheusConverter) addExponentialHistogramDataPoints(dataPoints pmetric.ExponentialHistogramDataPointSlice,
 	resource pcommon.Resource, settings Settings, promName string) error {
 	for x := 0; x < dataPoints.Len(); x++ {