[Exporter.Prometheus] Reduce allocations

[martincostello]

Changes

Reduce allocations by caching scope labels and their data for each metric.

Discovered while benchmarking v1.15.2 against v1.16.0.

Builds on top of #7440.

See #7465 (comment) for benchmark results

Old benchmark results

Indicative improvements:

Prometheus serializer: allocated bytes per call (net10.0, Default job)

Benchmark	1.15.2	1.16.0	PR #7440	#7440 + this	this vs 1.16.0	this vs #7440
WriteMetric (4 metrics)	104	4,856	3,360	736	−84.8%	−78.1%
WriteHistogramMetric	0	1,448	1,128	472	−67.4%	−58.2%
WriteMetricWithTypedLabels	104	1,344	744	88	−93.5%	−88.2%

Prometheus serializer: time per call in ns (net10.0, Default job, @1000 scale)

Benchmark	1.15.2	1.16.0	PR #7440	#7440 + this	this vs 1.16.0	this vs 1.15.2
WriteMetric	3,364	3,285	3,165	2,536	−22.8%	−24.6%
WriteHistogramMetric	2,804	1,667	1,670	1,527	−8.4%	−45.5%
WriteMetricWithTypedLabels	278	565	515	469	−17.0%	+68.7%

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 90.28%. Comparing base (68c29d8) to head (d55091b).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7465      +/-   ##
==========================================
+ Coverage   90.18%   90.28%   +0.09%     
==========================================
  Files         287      281       -6     
  Lines       15553    15031     -522     
==========================================
- Hits        14027    13571     -456     
+ Misses       1526     1460      -66     

Flag	Coverage Δ
unittests-PowerShellScripts	?
unittests-Project-Experimental	90.13% <100.00%> (-0.08%)	⬇️
unittests-Project-Stable	90.13% <100.00%> (-0.11%)	⬇️
unittests-UnstableCoreLibraries-Experimental	50.58% <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	99.49% <100.00%> (+<0.01%)	⬆️

... and 13 files with indirect coverage changes

[martincostello]

Just need to re-run the benchmarks after merging latest changes.

[martincostello]

Allocations (deterministic — the metric this change targets)

PrometheusSerializerBenchmarks (per serialize call):

Method	main	branch	Δ
WriteMetric	3,360 B	736 B	−78%
WriteHistogramMetric	1,128 B	472 B	−58%
WriteMetricWithTypedLabels	744 B	88 B	−88%

PrometheusSerializerEscapingBenchmarks (allocation per WriteMetrics call, summing 4 metrics), all 16 format × escaping combos:

Format	main	branch	Δ
PrometheusText V0 (all schemes)	4.72 KB	1.52 KB	−68%
PrometheusText V1	4.72–4.89 KB	1.52–1.69 KB	−65% to −68%
OpenMetrics V0	6.93 KB	1.80 KB	−74%
OpenMetrics V1	6.99–7.16 KB	1.80–1.98 KB	−72% to −74%

A consistent ~3.2 KB drop per call across every format and escaping scheme — confirming the caching removes the per-call scope-label collection rebuilds regardless of the negotiated escaping, exactly as intended.

[Copilot]

Pull request overview

This PR reduces Prometheus text serialization allocations by caching the computed otel_scope_* label collections per Metric, so they can be reused across repeated WriteMetric calls rather than rebuilt every time.

Changes:

• Added ConditionalWeakTable caches for scope label LabelData and the merged scope label key/value pairs, keyed by Metric.
• Switched serialization paths to use cached scope labels via GetScopeLabelData / GetScopeLabels.
• Introduced small helper methods to encapsulate cache access while keeping existing scope-label construction logic intact.

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[Kielek]

Note for myself: [] is not working on .NET Fx for this case.