[Exporter.Prometheus] Improve UTF-8 escaping performance

[martincostello]

Changes

Improve the performance of escaping UTF-8 values during serialization to be O(n) instead of O(n^2) when targeting modern .NET.

Benchmarks

Case	Before	After	Delta
NonAsciiLong (~3.6k chars)	86.4 µs	16.0 µs	~5.4x faster
AsciiLong (~980 chars)	166 ns	90 ns	~1.8x faster
NonAsciiShort (~18 chars)	75.7 ns	106.8 ns	within noise
AsciiShort (~11 chars)	10.8 ns	19.6 ns	within noise
Allocated (all cases)	0 B	0 B	no change

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 91.85%. Comparing base (6ee4b19) to head (20606e5).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7758      +/-   ##
==========================================
- Coverage   91.87%   91.85%   -0.03%     
==========================================
  Files         339      339              
  Lines       18468    18469       +1     
==========================================
- Hits        16967    16964       -3     
- Misses       1501     1505       +4     

Flag	Coverage Δ
unittests-Project-Experimental	91.94% <100.00%> (-0.05%)	⬇️
unittests-Project-Stable	91.96% <100.00%> (-0.05%)	⬇️
unittests-UnstableCoreLibraries-Experimental	50.48% <100.00%> (+<0.01%)	⬆️

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	98.71% <100.00%> (+<0.01%)	⬆️

... and 3 files with indirect coverage changes

[Copilot]

🟡 Changes recommended

The performance claim needs before-and-after benchmark results.

Get a fresh assessment by requesting another Copilot review.

Pull request overview

Optimizes shared Prometheus UTF-8 escaping by scanning input once.

Changes:

• Replaces dual searches with one verbatim-character search.
• Documents the optimization for both exporter packages.

File summaries

File	Description
TextFormatSerializer.cs	Implements linear escaping scan.
HttpListener CHANGELOG.md	Records the optimization.
AspNetCore CHANGELOG.md	Records the shared optimization.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Balanced

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[opentelemetry-pr-dashboard]

Pull request dashboard status

Merged · refreshed 2026-09-18 10:16 UTC

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