Support partial write on PRW2.0

[SungJin1212]

Implements partial write for Prometheus Remote Write 2.0 requests, following the Prometheus receiver behavior.
Previously a single invalid series aborted the whole conversion, so the entire batch was rejected with a 400 and the X-Prometheus-Remote-Write-*-Written response headers were not set at all; now invalid series are skipped and reported together in the response, the valid ones are written, and the written stats headers are always set.

This also makes Cortex accept a v2 write request that contains an exemplar-only series.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]
• docs/configuration/v1-guarantees.md updated if this PR introduces experimental flags

[yeya24]

Previously a single invalid series aborted the whole conversion, so the entire batch was rejected with a 400 and the X-Prometheus-Remote-Write-*-Written response headers were not set at all; now invalid series are skipped and reported together in the response, the valid ones are written, and the written stats headers are always set.

Interesting, is this what Prometheus does today? If that's Prometheus' behavior I am ok with it. But it is a behavior change in Cortex regardless. Do you think we can put this behind a FF?

[SungJin1212]

Interesting, is this what Prometheus does today?

Yes, Prometheus' v2 receiver has addressed the partial write since v2.54.0.

Do you think we can put this behind a FF?

I'd rather not add another flag here: the v2 path is already opt-in and experimental. Currently, Prometheus can send exemplar-only series (prometheus/prometheus#17857), and Cortex drops the whole request, so partial write is what makes exemplars usable over RW2.0 at all.
Or how about a flag that defaults to true?