[Exporter.Prometheus] Add target_info fallback

[martincostello]

Changes

Add Prometheus text fallback target_info output as a gauge so resource metadata is still exposed as Info-typed metrics are unavailable for PrometheusText exposition format.

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 89.92%. Comparing base (f3e872f) to head (1cb025e).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7238      +/-   ##
==========================================
+ Coverage   89.86%   89.92%   +0.05%     
==========================================
  Files         276      276              
  Lines       14007    14022      +15     
==========================================
+ Hits        12588    12609      +21     
+ Misses       1419     1413       -6     

Flag	Coverage Δ
unittests-Project-Experimental	89.61% <100.00%> (-0.22%)	⬇️
unittests-Project-Stable	89.85% <100.00%> (+<0.01%)	⬆️
unittests-UnstableCoreLibraries-Experimental	47.22% <100.00%> (+0.04%)	⬆️

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	91.73% <100.00%> (+0.13%)	⬆️
...heus.HttpListener/Internal/PrometheusSerializer.cs	98.78% <100.00%> (+0.05%)	⬆️

... and 6 files with indirect coverage changes

[Copilot]

Pull request overview

Adds a Prometheus text/plain fallback for resource metadata by emitting target_info as a gauge (value 1) when Info-typed metric families aren’t available in the Prometheus text exposition format.

Changes:

• Emit target_info as # TYPE target_info gauge / # HELP target_info ... for Prometheus text output while keeping OpenMetrics # TYPE target info.
• Cache target_info serialization separately for plain text vs OpenMetrics responses.
• Update HttpListener and AspNetCore integration tests + exporter changelogs to reflect the new plaintext output.

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/PrometheusHttpListenerTests.cs	Updates expected scrape output to include plaintext target_info gauge.
test/OpenTelemetry.Exporter.Prometheus.AspNetCore.Tests/PrometheusExporterMiddlewareTests.cs	Updates expected scrape output to include plaintext target_info gauge.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializer.cs	Makes WriteTargetInfo switch between OpenMetrics info vs plaintext gauge family names.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusCollectionManager.cs	Writes/caches target_info for both formats and splits cached lengths by format.
src/OpenTelemetry.Exporter.Prometheus.HttpListener/CHANGELOG.md	Documents the fallback behavior addition.
src/OpenTelemetry.Exporter.Prometheus.AspNetCore/CHANGELOG.md	Documents the fallback behavior addition.

Comments suppressed due to low confidence (1)

src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializer.cs:472

• WriteTargetInfo always appends a comma for each resource attribute and then unconditionally does cursor-- to overwrite the last comma. If resource is not reference-equal to Resource.Empty but has zero attributes (e.g., Resource.Empty.Merge(Resource.Empty) returns a new empty Resource), this will overwrite the { and emit invalid exposition (target_info} 1). Track whether any labels were written (or check for an empty attribute collection) and either omit braces/commas or skip emitting the metric when there are no attributes.

        cursor = WriteAsciiStringNoEscape(buffer, cursor, "target_info");
        buffer[cursor++] = unchecked((byte)'{');

        foreach (var attribute in resource.Attributes)
        {
            cursor = WriteLabel(buffer, cursor, attribute.Key, attribute.Value);

            buffer[cursor++] = unchecked((byte)',');
        }

        cursor--; // Write over the last written comma

        buffer[cursor++] = unchecked((byte)'}');

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Kielek]

From Codex:
[P2] src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusCollectionManager.cs:238 now calls WriteTargetInfo for the default text/plain path too. WriteTargetInfo only skips the Resource.Empty singleton, so a reachable empty-but-not-singleton resource, for example one returned by a detector or built from empty attributes, writes no labels and then cursor-- overwrites {, producing invalid exposition like target_info} 1. This was already risky for OpenMetrics, but this PR extends it to the default scrape format. Guard on empty resource.Attributes or handle the no-label sample explicitly, and add a plaintext regression test.

[martincostello]

Feedback addressed.