[Exporter.Prometheus] Export _created series for OpenMetrics

[martincostello]

Changes

Export {name}_created series for counters and histograms when start time is available when using OpenMetrics.

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 97.22222% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.94%. Comparing base (f3e872f) to head (91de284).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...s.HttpListener/Internal/PrometheusSerializerExt.cs	96.96%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7223      +/-   ##
==========================================
+ Coverage   89.86%   89.94%   +0.07%     
==========================================
  Files         276      276              
  Lines       14007    14043      +36     
==========================================
+ Hits        12588    12631      +43     
+ Misses       1419     1412       -7     

Flag	Coverage Δ
unittests-Project-Experimental	89.82% <97.22%> (-0.02%)	⬇️
unittests-Project-Stable	89.78% <97.22%> (-0.07%)	⬇️
unittests-UnstableCoreLibraries-Experimental	47.33% <97.22%> (+0.16%)	⬆️

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

Files with missing lines	Coverage Δ
...heus.HttpListener/Internal/PrometheusSerializer.cs	98.74% <100.00%> (+0.01%)	⬆️
...s.HttpListener/Internal/PrometheusSerializerExt.cs	98.40% <96.96%> (-0.52%)	⬇️

... and 5 files with indirect coverage changes

[Copilot]

Pull request overview

Adds support for exporting OpenMetrics {name}_created series for counters and histograms when a metric start time is available, and updates tests/changelogs accordingly.

Changes:

• Emit {name}_created samples for counters and histograms in OpenMetrics format (when MetricPoint.StartTime is present).
• Extend serializer and integration tests to validate presence/absence of _created depending on OpenMetrics negotiation.
• Update Prometheus exporter changelogs to document the new behavior.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusSerializerTests.cs	Adds assertions that _created is exported only for OpenMetrics (counter + histogram).
test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusHttpListenerTests.cs	Updates HTTP listener integration expectations to include _created in OpenMetrics responses.
test/OpenTelemetry.Exporter.Prometheus.AspNetCore.Tests/PrometheusExporterMiddlewareTests.cs	Updates ASP.NET Core middleware integration expectations to include _created in OpenMetrics responses.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializerExt.cs	Implements writing of {name}_created lines based on MetricPoint.StartTime.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/CHANGELOG.md	Documents the new _created OpenMetrics series behavior.
src/OpenTelemetry.Exporter.Prometheus.AspNetCore/CHANGELOG.md	Documents the new _created OpenMetrics series behavior.

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[rajkumar-rangaraj]

@martincostello could you please check the copilot cli feedback.

PR #7223 Review: [Exporter.Prometheus] Export _created series for OpenMetrics

🔴 CRITICAL: Incorrect # TYPE _created gauge metadata line

The PR emits a separate # TYPE {name}_created gauge metadata declaration. However, per the OpenMetrics spec,
_created is a suffix/sample within the parent MetricFamily, NOT a separate MetricFamily. None of the spec examples
show a separate TYPE line for _created:

Spec example (Counter) — NO separate TYPE for _created:

TYPE foo counter

foo_total 17.0
foo_created 1520430000.123

The PR incorrectly emits:

TYPE foo_created gauge ← NOT in any spec example

TYPE foo counter

foo_total 17.0
foo_created 1520430000.123

This could cause Prometheus parsers to reject the output since they know _created is a special suffix, not a
standalone gauge.

🟡 MEDIUM: Timestamp precision

WriteUnixTimeSeconds uses ToUnixTimeMilliseconds() / 1000.0 → only millisecond precision. The WriteDouble with G17
format may produce verbose representations. Functional but worth documenting.

🟢 LOW: Double iteration of MetricPoints

HasCreatedMetric() iterates all metric points to check for non-default StartTime, then the main loop iterates again.
GetMetricPoints() returns a new MetricPointsAccessor struct each time, so it's correct but slightly inefficient.

✅ Verified OK

• _created correctly excluded for gauges (only counters + histograms)
• WriteCreatedMetric correctly guards with StartTime != default per-point
• Non-OpenMetrics paths correctly use empty string for createdMetric
• HasCreatedMetric double-iteration is safe (enumerator is value-type, reusable)
• All CI checks pass

The # TYPE {name}_created gauge metadata line appears to violate the OpenMetrics spec. The rest of the implementation looks solid.

[martincostello]

Will do - the fun of Copilot. Changes originally authored by it won't always fulfill its own review analysis 😄

[Kielek]

Codex feedback:
[P2] src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializerExt.cs:134 ties histogram _created emission to the same block that emits _sum / _count. For OpenMetrics histograms with negative bucket bounds, this block is skipped, so _created is also skipped. The spec only forbids _sum in that case; it still says histogram points should contain Created, and the text format defines _created independently of _sum / _count. Move the WriteCreatedMetric(...) call outside that !hasNegativeBucketBounds block so it still emits after the buckets when StartTime is available.

[martincostello]

Done.