histograms: Implement final OM text format

[beorn7]

Once OM has decided how the text format representation of Native Histograms should look like (likely in OM 2.0, cf. prometheus/OpenMetrics#237 for tracking), we need to implement it here in prometheus/prometheus.

This is needed to declare Native Histograms a stable feature.

See also #11172, which will inform how to design the text format.

[beorn7]

For reference: https://docs.google.com/document/d/1w6GhLmDYKkkNLsyPWkC3TGhW3Rs1G73j7E_nG7bhlw0/edit# enumerates ideas how an OM text format could look like

[weyert]

Are there any decisions made on the OM text format?

I would interested in implementing the text format for the OpenTelemetry Prometheus exporter in otel-js.

[beorn7]

No decisions made yet. The next step would be to create a prototype in any of the instrumentation libraries based on the brainstorming doc linked above. https://github.com/prometheus/client_python comes to mind, and @csmarchbanks seemed interesting in exploring the topic. Not sure how soon he'll have time (or if he is ready to receive PRs from others).