Support Prometheus native histograms

[dashpole]

Problem Statement

We currently drop exponential histograms in the prometheus exporter:

opentelemetry-go/exporters/prometheus/exporter.go

Lines 226 to 239 in 48fedfa

	switch v := m.Data.(type) {
	case metricdata.Histogram[int64]:
	addHistogramMetric(ch, v, m, keys, values, name, c.resourceKeyVals)
	case metricdata.Histogram[float64]:
	addHistogramMetric(ch, v, m, keys, values, name, c.resourceKeyVals)
	case metricdata.Sum[int64]:
	addSumMetric(ch, v, m, keys, values, name, c.resourceKeyVals)
	case metricdata.Sum[float64]:
	addSumMetric(ch, v, m, keys, values, name, c.resourceKeyVals)
	case metricdata.Gauge[int64]:
	addGaugeMetric(ch, v, m, keys, values, name, c.resourceKeyVals)
	case metricdata.Gauge[float64]:
	addGaugeMetric(ch, v, m, keys, values, name, c.resourceKeyVals)
	}

Proposed Solution

We should convert them to prometheus native histograms per https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/compatibility/prometheus_and_openmetrics.md#exponential-histograms

Additional Context

Related to open-telemetry/opentelemetry-collector-contrib#33703

This is currently difficult to do because we use prometheus.NewConstHistogram here:

opentelemetry-go/exporters/prometheus/exporter.go

Line 256 in 48fedfa

m, err := prometheus.NewConstHistogram(desc, dp.Count, float64(dp.Sum), buckets, values...)

There is no NewConstNativeHistogram in the prometheus client. We would currently need to implement the prometheus.Metric interface directly, which is a bit more work.

[dashpole]

I opened prometheus/client_golang#1617 to see if the prometheus client is interested in adding a const function for this use-case.

[dashpole]

This is mostly unblocked after prometheus/client_golang#1654. We just need to wait for a release. Exemplars are not yet supported

[ajatprabha]

I see that prometheus/client_golang#1654 is now merged, is this being worked on somewhere now[issue/pr]?

[GiedriusS]

I started working on this, about to open up a PR. Are you working on this too?

[ajatprabha]

No, not yet, wanted to confirm before starting to work on a PR.

I'll wait for your PR, TIA!

[GiedriusS]

@ajatprabha opened #6421. PTAL.