fix: add missing variant dimension in MetricsHook metrics

[askpt]

Summary

Fixes #741

The MetricsHook.AfterAsync method was not including feature_flag.result.variant as a dimension/tag on the feature_flag.evaluation_success_total metric. This is inconsistent with:

• The Java SDK
• The Go SDK
• The OpenTelemetry semantic conventions

Changes

• src/OpenFeature/Hooks/MetricsHook.cs: Added details.Variant to the tag list in AfterAsync, guarded by a null check (matching the Java SDK pattern). Uses the existing TelemetryConstants.Variant constant.
• test/OpenFeature.Tests/Hooks/MetricsHookTests.cs:
  • Updated After_Test to assert the variant tag is present on success metrics.
  • Added Without_Variant_After_Test_Does_Not_Include_Variant test to verify the tag is omitted when variant is null.

Notes

Metrics now include the missing variant. See screenshot:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.47%. Comparing base (98c394f) to head (6f9c8e9).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #742   +/-   ##
=======================================
  Coverage   93.46%   93.47%           
=======================================
  Files          68       68           
  Lines        2952     2956    +4     
  Branches      350      351    +1     
=======================================
+ Hits         2759     2763    +4     
  Misses        135      135           
  Partials       58       58           

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[gemini-code-assist]

Code Review

This pull request updates the MetricsHook to include the evaluation variant in telemetry tags when it is available. It also includes corresponding unit tests to verify that the variant tag is correctly added when present and omitted when null. I have no feedback to provide.