[Exporter.Prometheus] Fix scope metadata

[martincostello]

Changes

Scope-related fixes for OpenMetrics and Prometheus specification compliance.

• Include instrumentation scope metadata on samples using otel_scope_* labels, including scope version, schema URL, and prefixed scope attributes.
• OpenMetrics output no longer emits a separate otel_scope_info scope metadata metric.
• Drop conflicting scope attributes named name, version, and schema_url to avoid collisions with generated scope labels.

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 93.75000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.03%. Comparing base (78f6d59) to head (06d421f).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...heus.HttpListener/Internal/PrometheusSerializer.cs	93.44%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7237      +/-   ##
==========================================
- Coverage   90.04%   90.03%   -0.02%     
==========================================
  Files         276      276              
  Lines       14271    14283      +12     
==========================================
+ Hits        12850    12859       +9     
- Misses       1421     1424       +3     

Flag	Coverage Δ
unittests-Project-Experimental	89.92% <93.75%> (-0.02%)	⬇️
unittests-Project-Stable	89.84% <93.75%> (-0.09%)	⬇️
unittests-UnstableCoreLibraries-Experimental	48.62% <93.75%> (+0.09%)	⬆️

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

Files with missing lines	Coverage Δ
...tpListener/Internal/PrometheusCollectionManager.cs	93.52% <100.00%> (+1.21%)	⬆️
...heus.HttpListener/Internal/PrometheusSerializer.cs	97.65% <93.44%> (+0.13%)	⬆️

... and 3 files with indirect coverage changes

[Copilot]

Pull request overview

This PR updates the Prometheus/OpenMetrics exporter serialization to be compliant with OpenMetrics/Prometheus scope metadata expectations by emitting scope metadata once per scrape and attaching instrumentation scope details to metric samples.

Changes:

• Emit OpenMetrics scope metadata as a single otel_scope metric family (with otel_scope_info samples) instead of repeating TYPE/HELP per scope.
• Add instrumentation scope metadata onto metric samples using otel_scope_* labels (version, schema URL, and prefixed scope attributes).
• Update/extend unit + integration tests and exporter changelogs to reflect the new output.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusSerializerTests.cs	Updates serializer expectations and adds tests for scope-label prefixing and reserved-name dropping.
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusHttpListenerTests.cs	Updates integration expected output for new scope metric family + prefixed scope attributes.
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusCollectionManagerTests.cs	Adds coverage ensuring OpenMetrics scope metadata is written once as a single metric family.
test/OpenTelemetry.Exporter.Prometheus.AspNetCore.Tests/PrometheusExporterMiddlewareTests.cs	Updates middleware integration expectations for new scope metric family + prefixed scope attributes.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializerExt.cs	Adjusts histogram bucket serialization to account for WriteTags no longer leaving a trailing comma.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializer.cs	Implements scope-metadata family split (metadata vs samples) and writes scope labels (version/schema/tags) with otel_scope_ prefix.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusCollectionManager.cs	Emits scope metadata once and deduplicates scope identities across metrics during OpenMetrics scrapes.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/CHANGELOG.md	Documents the scope-metadata and scope-label behavior changes.
src/OpenTelemetry.Exporter.Prometheus.AspNetCore/CHANGELOG.md	Documents the scope-metadata and scope-label behavior changes.

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[Kielek]

From Codex:

src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializer.cs:664
Scope attribute label collisions are not handled. TryCreateScopeLabel normalizes each scope attribute independently, so tags like library.mascot and library-mascot both serialize as otel_scope_library_mascot, producing duplicate label names. The OTel Prometheus compatibility spec says colliding converted labels must be concatenated with ;, ordered by original key.

src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusCollectionManager.cs:264
The synthetic otel_scope metric family bypasses existing metadata conflict handling. If user code exports a metric whose OpenMetrics metadata name is otel_scope, the scrape can contain both # TYPE otel_scope info and the user metric’s # TYPE otel_scope ..., violating the existing “no duplicate/conflicting TYPE comments” rule. This should be reserved/seeded into the same conflict logic that handles real metrics.

[martincostello]

Both comments addressed.

[Kielek]

2 new codex findings:
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusCollectionManager.cs:422
The new reservation only drops user metrics whose OpenMetrics metadata name is exactly otel_scope, but the synthetic info family also emits the sample name otel_scope_info. A user metric such as otel.scope.info still sanitizes to a separate otel_scope_info family and emits an otel_scope_info{...} sample, clashing with the synthetic info sample. OpenMetrics explicitly calls out suffix-based sample-name clashes for info families, so this should reserve/drop otel_scope_info as well when scope metadata is emitted.

src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializer.cs:441
Scope labels are built and written before metric point tags, but they are not merged with the point labels. A scope attribute like library.mascot produces otel_scope_library_mascot; a metric point attribute already named otel_scope_library_mascot will then produce duplicate label names in the same label set. The OTel compatibility spec says scope attributes follow the Metric Attributes rules, and collisions with labels added by the spec must concatenate values with ;.

[Kielek]

One more findings against spec complianace:

src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializer.cs:725
CreateMetricPointLabels now concatenates point-tag collisions with existing scope labels, but it always appends the point value after the existing value. The OTel Prometheus compatibility spec requires colliding values to be ordered by the lexicographical order of the original keys. This works for the current test case, but fails for cases like scope attribute z="scope" plus point attribute otel_scope_z="point": both serialize to otel_scope_z, and the required order is point;scope, while this code emits scope;point.

This test could be changed to something like

[Theory]
[InlineData(false, "library.mascot", "dotnetbot", "otel_scope_library_mascot", "otter", "otel_scope_library_mascot", "dotnetbot;otter")]
[InlineData(true, "library.mascot", "dotnetbot", "otel_scope_library_mascot", "otter", "otel_scope_library_mascot", "dotnetbot;otter")]
[InlineData(false, "z", "scope", "otel_scope_z", "point", "otel_scope_z", "point;scope")]
[InlineData(true, "z", "scope", "otel_scope_z", "point", "otel_scope_z", "point;scope")]
public void WriteMetricConcatenatesPointTagsThatCollideWithScopeLabels(
    bool useOpenMetrics,
    string scopeTagKey,
    string scopeTagValue,
    string pointTagKey,
    string pointTagValue,
    string expectedLabelKey,
    string expectedLabelValue)
{

[martincostello]

Should be addressed now.

[Kielek]

FrozenSet should be more performant

[Kielek]

edit: alternatively, all these 3 items could be made a cosnt, and the comparision could be done by swithc/case/==. Depends on the level of optimization you need

[martincostello]

Added where available.

[Kielek]

LGTM with some minor comment. Auto merge disabled dur to this potential change.