[receiver/prometheusreceiver] Convert NHCB to explicit histogram metric

[jelly-afk]

Description

added prometheus NHCB (native histogram with custom buckets) to otel explicit histogram conversion.

Link to tracking issue

Fixes #41131

Testing

Added TestMetricGroupData_toNHCBDistributionUnitTest test function to test the conversion.

[jelly-afk]

I would like do some changes here, will open when done.

[jelly-afk]

In remotewrite receiver if spans are empty, bucket counts are not filled

opentelemetry-collector-contrib/receiver/prometheusremotewritereceiver/receiver.go

Lines 753 to 758 in 9076436

	bucketCounts := make([]uint64, len(histogram.CustomValues)+1)
	// NHCB uses the positive bucket list and spans for all buckets
	if len(histogram.PositiveSpans) == 0 {
	return bucketCounts
	}

we should follow it here too, right?

[dashpole]

I believe it is filled with all zeroes by the make() line

[jelly-afk]

ready for review

[ArthurSens]

Thanks for working on this one! It looks almost ready, to be honest. I think we can improve a little bit on the readability of the getOrCreatedMetricFamily method, adding -1 in almost all calls of that function is looking weird 🤔

[ArthurSens]

Something feels off with this strategy of adding a schema as an argument 🤔

What if we took a similar approach to the one we use when detecting a NativeHistogram? When we call AppendHistogram, we toggle a flag in the transaction object called addingNativeHistogram. Couldn't we add the flag there, too?

[jelly-afk]

yes that sounds good too

[ArthurSens]

This is the place I had in mind in the comment above. Couldn't we add a second flag here, e.g.

	var schema int32
	if h != nil {
		schema = h.Schema
	} else if fh != nil {
		schema = fh.Schema
	}
	t.addingNativeHistogram = true
	if schema == -53 {
		t.addingNHCB = true
	}

If we use that flag when calling getOrCreateMetricFamily, then the schema argument isn't needed and we don't need to add -1 in weird places 😅

[ArthurSens]

I believe this could become a single if...?

[jelly-afk]

yes, missed that

[jelly-afk]

oh changelog got new validations, will fix

[jelly-afk]

ah chlog files from prev release

[ArthurSens]

LGTM, but I'd love it if we could wait for @dashpole or @krajorama's review as well

[dashpole]

huh, something strange happened with my comments. Maybe I didn't actually leave them? Ignore them for now

Edit: Re-posted it below

[otelbot]

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