[dashpole]

Allow Summary metrics in OpenMetrics 2.0 to expose negative Sum and Quantile values when negative event values are observed:

• Clarify that Count is a counter semantically (MUST NOT be NaN or negative and MUST be an integer), while Sum MUST NOT be NaN and is only a counter semantically as long as there are no negative event values measured by the Summary (matching the existing Histogram wording).
• Remove the Quantile values MUST NOT be negative. restriction so summaries observing negative values can expose negative quantile values.

This aligns the specification with existing instrumentation libraries (such as client_golang and client_java) and Prometheus's OpenMetrics parsers (see prometheus/prometheus#19894 (comment)).

[Copilot]

🟢 Approval recommended

Both specification changes match the stated intent, with no unresolved review findings.

No open issues 🎉

What changed in this PR

Updates OpenMetrics 2.0 Summary semantics to support negative observations.

Changes:

• Allows negative Sum values while preserving Count constraints and prohibiting NaN sums.
• Removes the restriction against negative quantile values.

File	Description
docs/​specs/​om/​open_metrics_spec_2_0.md	Revises Summary constraints to permit negative sums and quantile values.

_{Review effort: Balanced}

[krajorama]

LGTM, we're still different from Histograms in that NaN is not allowed in Sum, but let's discuss that separately.