[Exporter.Prometheus] Fix number formatting

[martincostello]

Fixes #7585

Changes

Fix incorrectly formatted floating point numbers caused by a misunderstanding of Canonical Numbers.

Specifically this part:

A warning to implementers in C and other languages that share its printf implementation: The standard precision of %f, %e and %g is only six significant digits. 17 significant digits are required for full precision, e.g. printf("%.17g", d).

I took it to mean to use G17 to format things, but that over-specifies the precision and causes non-exact round-tripping issues as described in the linked discussion.

Re-running the benchmarks locally don't show any regression in terms of runtime or allocation due to the use of stackalloc.

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)

[opentelemetry-pr-dashboard]

Pull request dashboard status

Merged · refreshed 2026-08-12 22:39 UTC

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.52%. Comparing base (312c1df) to head (708707f).
⚠️ Report is 17 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7589   +/-   ##
=======================================
  Coverage   90.51%   90.52%           
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  Files         292      292           
  Lines       16197    16215   +18     
=======================================
+ Hits        14661    14678   +17     
- Misses       1536     1537    +1     

Flag	Coverage Δ
unittests-PowerShellScripts	87.43% <ø> (ø)
unittests-Project-Experimental	90.61% <100.00%> (+0.02%)	⬆️
unittests-Project-Stable	90.59% <100.00%> (+0.16%)	⬆️
unittests-Solution	90.59% <100.00%> (+0.02%)	⬆️
unittests-UnstableCoreLibraries-Experimental	51.18% <100.00%> (+0.08%)	⬆️

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

Files with missing lines	Coverage Δ
...ernal/Shared/Serialization/TextFormatSerializer.cs	98.81% <100.00%> (+0.02%)	⬆️

... and 3 files with indirect coverage changes

[Copilot]

Pull request overview

This PR fixes floating-point number formatting in the Prometheus exporters’ text serialization to use the shortest round-trippable representation for sample values, and to correctly render OpenMetrics canonical numbers for histogram le and summary quantile label values. It updates unit/integration/fuzz tests and changelogs to lock in the corrected behavior and prevent regressions.

Changes:

• Updated Prometheus text serialization to avoid G17-style over-precision padding and to correctly render canonical numbers for le/quantile.
• Added/updated unit, integration, fuzz tests and snapshot baselines to validate round-tripping and boundary rendering.
• Added CHANGELOG entries for both Prometheus.HttpListener and Prometheus.AspNetCore packages.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/snapshots/PrometheusSerializer.HistogramWithFractionalBoundaries_useOpenMetrics=True.verified.txt	New snapshot baseline for histogram fractional boundary rendering (OpenMetrics).
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/snapshots/PrometheusSerializer.HistogramWithFractionalBoundaries_useOpenMetrics=False.verified.txt	New snapshot baseline for histogram fractional boundary rendering (Prometheus text).
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusSerializerTests.cs	Updates numeric formatting expectations and adds regression tests for round-tripping/precision.
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusIntegrationTests.cs	Adjusts integration test bucket boundaries to cover more rendered shapes.
test/OpenTelemetry.Exporter.Prometheus.HttpListener.FuzzTests/PrometheusSerializerFuzzTests.cs	Updates fuzz property to validate round-tripping without unnecessary padding.
test/OpenTelemetry.Exporter.Prometheus.AspNetCore.Tests/PrometheusIntegrationTests.cs	Adds assertions to validate bucket boundary round-tripping through Prometheus API.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/Shared/Serialization/TextFormatSerializer.cs	Core implementation change: shortest round-trippable formatting and corrected canonical number rendering.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/CHANGELOG.md	Adds Unreleased entries describing formatting/canonical-number fixes.
src/OpenTelemetry.Exporter.Prometheus.AspNetCore/CHANGELOG.md	Adds Unreleased entries describing formatting/canonical-number fixes.

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