Prometheus exporter: Stabilize section 'Content Negotiation'

[ArthurSens]

Fixes #4991

Changes

Stabilize the section Content Negotiation in the Prometheus exporter spec.

While reviewing the SDK implementation, I found these:

Fully compliant

• Go SDK - Let's Prometheus Go SDK handle content negotiation; it is fully compliant.
• Java SDK - Owns Content negotiation, but let's Prometheus Java SDK handle the transformation of a Registry into the output. Is fully compliant.
• Collector's Prometheus exporter - Let's Prometheus Go SDK handle content negotiation; It is fully compliant.

Almost compliant

• Python SDK - Let's Prometheus Python SDK handle content negotiation, but Prometheus Python SDK doesn't honor q weights and doesn't support protobuf.
• DotNet - It parses Accept headers, supports text/plain and application/openmetrics, respects q weights, and falls back to Prometheus text 0.0.4. It doesn't support. allow-utf-8 is treated the same way as underscores, they are always escaped.

Non compliant

• JS SDK - No content negotiation.
• C++ SDK - No content negotiation.
• Rust SDK - No content negotiation.
• Swift SDK - No content negotiation.

---

• Related issues Stabilize Prometheus Metrics Exporter spec: Content Negotiation #4991
• Related OTEP(s) #
• Links to the prototypes (when adding or changing features)
• CHANGELOG.md file updated for non-trivial changes
  • For trivial changes, include [chore] in the PR title to skip the changelog check
• Spec compliance matrix updated if necessary
• Declarative config data model is updated if SDK config surface is changed

[krajorama]

lgtm

[krajorama]

Actually after reading #5062 , it seems like we should specify what to do if no Accept header is presented by the client to make it explicit. Fall back to text/plain version=0.0.4 and underscores translation. Text format 0.0.4 doesn't support utf-8.

[krajorama]

Also there's a dependency to #5134 as that modifies the content negotiation part.