Datasources: Enable native histograms for datasource response size metric

[marefr]

What is this feature?

Enables native histogram for the grafana_datasource_response_size_bytes metric.

If Prometheus feature flag native-histograms

• is not enabled (default) everything will continue to work as today.
• is enabled only the native histogram will be scraped and classic histograms ignored unless scrape of classic histogram is enabled in the scrape config then you'll get both the native and the classic ones scraped.

Why do we need this feature?

To be able to gather more valuable information about maximum/average response sizes. The current buckets only tracks up to 1MB and is to limited to understand what distribution goes between 1MB and infinity.

Who is this feature for?

Operators of Grafana.

Which issue(s) does this PR fix?:
Ref #35796

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[xnyo]

LGTM! 🚀

[bergquist]

LGTM

[marefr]

/deploy-to-hg

[ephemeral-instances-bot]

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[ephemeral-instances-bot]

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[marefr]

Seems to work as expected in ephemeral instance scraping both native and classic histogram 🎉