[Exporter.Prometheus] Add allow-utf-8 escaping

[martincostello]

Fixes #7246

Changes

Add support for the allow-utf-8 escaping scheme.

Benchmark Results

Summary (generated by Claude 🐙)

Prometheus serializer allocation improvements (this branch vs gh-7246)

PrometheusSerializerBenchmarks (Prometheus text v1.0.0 / underscores), BenchmarkDotNet Default job + memory diagnoser, .NET 10.0.9, i7-13700H. Both refs benchmarked in the same session (gh-7246 from a fresh worktree; this branch = allow-utf-8 + label-key allocation optimizations). Allocation normalised to KB per single op.

Benchmark	Metric	gh-7246	This branch	Δ
WriteMetric	Allocated	4.12 KB	3.28 KB	−20.4%
	Mean (×1)	2,582 ns	2,485 ns	−3.7%
	Mean (×10000)	26.30 ms	25.24 ms	−4.0%
WriteHistogramMetric	Allocated	1.26 KB	1.10 KB	−12.5%
	Mean (×10000)	14.41 ms	14.05 ms	−2.5%
WriteMetricWithTypedLabels	Allocated	1.16 KB	0.73 KB	−37.3%
	Mean (×10000)	4.47 ms	3.90 ms	−12.7%

Allocation scales linearly across the 1 / 1000 / 10000 call-count params, confirming the per-op figures.

Summary: allocation is down 13–37% in core serialization with neutral-to-faster throughput (StdDevs ~1% this run). The reduction comes from no longer allocating a StringBuilder + string for every output label key — already-valid keys are now returned/written with zero allocation. The allow-utf-8 feature itself is allocation-neutral on the common path.

Detailed Results

Expand to view

#7439

BenchmarkDotNet v0.15.8, Windows 11 (10.0.26200.8655/25H2/2025Update/HudsonValley2)
13th Gen Intel Core i7-13700H 2.90GHz, 1 CPU, 20 logical and 14 physical cores
.NET SDK 10.0.301
  [Host]     : .NET 10.0.9 (10.0.9, 10.0.926.27113), X64 RyuJIT x86-64-v3
  DefaultJob : .NET 10.0.9 (10.0.9, 10.0.926.27113), X64 RyuJIT x86-64-v3

Method	NumberOfSerializeCalls	Mean	Error	StdDev	Gen0	Allocated
WriteMetric	1	2,581.5 ns	27.57 ns	23.03 ns	0.3357	4.12 KB
WriteHistogramMetric	1	1,481.5 ns	29.18 ns	28.66 ns	0.1011	1.26 KB
WriteMetricWithTypedLabels	1	441.4 ns	6.30 ns	5.89 ns	0.0939	1.16 KB
WriteMetric	1000	2,572,046.6 ns	37,995.08 ns	31,727.61 ns	335.9375	4117.19 KB
WriteHistogramMetric	1000	1,475,238.5 ns	25,244.04 ns	23,613.29 ns	101.5625	1257.81 KB
WriteMetricWithTypedLabels	1000	433,982.6 ns	4,025.90 ns	3,143.16 ns	94.2383	1156.25 KB
WriteMetric	10000	26,298,293.8 ns	523,925.09 ns	643,427.12 ns	3343.7500	41171.88 KB
WriteHistogramMetric	10000	14,412,322.4 ns	259,791.33 ns	243,008.97 ns	1015.6250	12578.13 KB
WriteMetricWithTypedLabels	10000	4,467,305.4 ns	78,161.99 ns	69,288.56 ns	937.5000	11562.5 KB

This PR

BenchmarkDotNet v0.15.8, Windows 11 (10.0.26200.8655/25H2/2025Update/HudsonValley2)
13th Gen Intel Core i7-13700H 2.90GHz, 1 CPU, 20 logical and 14 physical cores
.NET SDK 10.0.301
  [Host]     : .NET 10.0.9 (10.0.9, 10.0.926.27113), X64 RyuJIT x86-64-v3
  DefaultJob : .NET 10.0.9 (10.0.9, 10.0.926.27113), X64 RyuJIT x86-64-v3

Method	NumberOfSerializeCalls	Mean	Error	StdDev	Gen0	Allocated
WriteMetric	1	2,485.1 ns	29.39 ns	24.54 ns	0.2670	3360 B
WriteHistogramMetric	1	1,417.2 ns	23.14 ns	21.65 ns	0.0896	1128 B
WriteMetricWithTypedLabels	1	397.9 ns	7.65 ns	6.38 ns	0.0591	744 B
WriteMetric	1000	2,519,263.3 ns	43,087.04 ns	40,303.65 ns	265.6250	3360000 B
WriteHistogramMetric	1000	1,361,864.6 ns	17,776.48 ns	16,628.13 ns	89.8438	1128000 B
WriteMetricWithTypedLabels	1000	391,383.2 ns	6,740.23 ns	8,023.77 ns	59.0820	744000 B
WriteMetric	10000	25,242,336.2 ns	329,968.13 ns	308,652.39 ns	2656.2500	33600000 B
WriteHistogramMetric	10000	14,048,737.1 ns	273,508.46 ns	242,458.12 ns	890.6250	11280000 B
WriteMetricWithTypedLabels	10000	3,901,919.2 ns	58,909.57 ns	55,104.05 ns	589.8438	7440000 B

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

❌ Patch coverage is 98.31933% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.41%. Comparing base (f4fd940) to head (5bfff05).
⚠️ Report is 11 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...ernal/Shared/Serialization/TextFormatSerializer.cs	97.74%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7440      +/-   ##
==========================================
+ Coverage   90.12%   90.41%   +0.28%     
==========================================
  Files         285      281       -4     
  Lines       15343    15414      +71     
==========================================
+ Hits        13828    13936     +108     
+ Misses       1515     1478      -37     

Flag	Coverage Δ
unittests-PowerShellScripts	?
unittests-Project-Experimental	90.10% <98.31%> (+0.02%)	⬆️
unittests-Project-Stable	90.21% <98.31%> (+0.07%)	⬆️
unittests-UnstableCoreLibraries-Experimental	50.55% <98.31%> (+0.87%)	⬆️

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

Files with missing lines	Coverage Δ
...metheus.AspNetCore/PrometheusExporterMiddleware.cs	100.00% <100.00%> (ø)
...s.HttpListener/Internal/PrometheusHeadersParser.cs	98.79% <100.00%> (-0.06%)	⬇️
...HttpListener/Internal/Shared/PrometheusEscaping.cs	100.00% <100.00%> (ø)
...s.HttpListener/Internal/Shared/PrometheusMetric.cs	99.58% <100.00%> (+0.04%)	⬆️
...HttpListener/Internal/Shared/PrometheusProtocol.cs	98.07% <100.00%> (+0.03%)	⬆️
...rnal/Shared/Serialization/OpenMetricsSerializer.cs	100.00% <100.00%> (ø)
...al/Shared/Serialization/OpenMetricsV1Serializer.cs	100.00% <100.00%> (ø)
...Shared/Serialization/PrometheusTextV1Serializer.cs	100.00% <100.00%> (ø)
...ernal/Shared/Serialization/TextFormatSerializer.cs	99.48% <97.74%> (-0.52%)	⬇️

... and 16 files with indirect coverage changes