[Exporter.Prometheus] Fix OpenMetrics histograms

[martincostello]

Changes

Omit histogram _sum and _count in OpenMetrics when negative bucket thresholds are present.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.92%. Comparing base (c0f7fb4) to head (7c58e78).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7221      +/-   ##
==========================================
- Coverage   89.98%   89.92%   -0.06%     
==========================================
  Files         272      272              
  Lines       13643    13649       +6     
==========================================
- Hits        12276    12274       -2     
- Misses       1367     1375       +8     

Flag	Coverage Δ
unittests-Project-Experimental	89.77% <100.00%> (-0.10%)	⬇️
unittests-Project-Stable	89.81% <100.00%> (-0.15%)	⬇️
unittests-UnstableCoreLibraries-Experimental	46.14% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...s.HttpListener/Internal/PrometheusSerializerExt.cs	98.91% <100.00%> (+0.04%)	⬆️

... and 4 files with indirect coverage changes

[Copilot]

Pull request overview

Fixes OpenMetrics histogram serialization in the Prometheus exporters to comply with the OpenMetrics spec when explicit bucket boundaries include negative thresholds.

Changes:

• Detect negative histogram bucket bounds during serialization when OpenMetrics output is requested.
• Omit histogram _sum and _count series in OpenMetrics output when negative bucket thresholds are present.
• Add a unit test validating _sum/_count omission and update changelogs for both Prometheus exporters.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusSerializerTests.cs	Adds coverage ensuring OpenMetrics output omits _sum and _count when buckets include negative bounds.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializerExt.cs	Implements conditional suppression of _sum/_count for OpenMetrics histograms with negative bucket thresholds.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/CHANGELOG.md	Documents the behavioral change in the HttpListener exporter changelog.
src/OpenTelemetry.Exporter.Prometheus.AspNetCore/CHANGELOG.md	Documents the behavioral change in the AspNetCore exporter changelog (shared serializer implementation).

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