Consider supporting constant native histograms

[dashpole]

For OpenTelemetry's prometheus exporters, we would like to be able to export OTel exponential histograms: open-telemetry/opentelemetry-collector-contrib#33703, open-telemetry/opentelemetry-go#5777.

It would be helpful to have a NewConstNativeHistogram constructor, which mirrors the NewConstHistogram constructor. Unclear if we need all of the variants (Vec, with CT, etc).

Alternatively, we could directly implement the prometheus.Metric interface. I don't think it would be that hard to do, but I haven't tried. Feel free to close this issue if we should go that route instead.

[ArthurSens]

I'm supportive :)

[Maniktherana]

I can give this a shot if thats okay

[beorn7]

As we still need to support exemplars on the const native histogram, I re-open this.

See #1654 (comment) for details how to implement that.

[beorn7]

#1686 fixed this.