[AspNetCore] Align route behaviour

[martincostello]

See martincostello#2 (comment).

Relates to #3808.

Changes

Align the behaviour for computing the route with the logic of ASP.NET Core 8+ itself.

The metric route is incorrectly empty in .NET 8 and 9 so the test data has been adjusted to account for that.

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 71.90%. Comparing base (22ad929) to head (e71f9a0).
⚠️ Report is 3 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3992      +/-   ##
==========================================
+ Coverage   71.75%   71.90%   +0.14%     
==========================================
  Files         458      448      -10     
  Lines       17869    17823      -46     
==========================================
- Hits        12822    12815       -7     
+ Misses       5047     5008      -39     

Flag	Coverage Δ
unittests-Instrumentation.AspNet	77.37% <ø> (ø)
unittests-Instrumentation.AspNetCore	70.09% <100.00%> (+0.45%)	⬆️
unittests-Instrumentation.Cassandra	?

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

Files with missing lines	Coverage Δ
...nstrumentation.AspNetCore/HttpContextExtensions.cs	100.00% <100.00%> (ø)

... and 10 files with indirect coverage changes

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[Copilot]

Pull request overview

Aligns http.route computation in the ASP.NET Core instrumentation with ASP.NET Core 8+ internal routing/diagnostics behavior, and updates route test expectations accordingly (including known .NET 8/9 metrics-route quirks).

Changes:

• Update route extraction to use IRouteDiagnosticsMetadata and normalize empty routes to /, mirroring ASP.NET Core behavior.
• Adjust route test case data and generated README outputs for .NET 8/9/10 to reflect the updated route/display-name behavior.
• Document the behavioral change in the ASP.NET Core instrumentation changelog.

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.Instrumentation.AspNetCore.Tests/RouteTests/RoutingTestCases.json	Updates expected/current route values for root path scenarios and .NET version-specific metrics expectations.
test/OpenTelemetry.Instrumentation.AspNetCore.Tests/RouteTests/README.net8.0.md	Updates recorded activity display name and route for RazorPages root path.
test/OpenTelemetry.Instrumentation.AspNetCore.Tests/RouteTests/README.net9.0.md	Updates recorded activity display name and route for RazorPages root path.
test/OpenTelemetry.Instrumentation.AspNetCore.Tests/RouteTests/README.net10.0.md	Updates recorded activity display name and route for RazorPages root path.
src/OpenTelemetry.Instrumentation.AspNetCore/HttpContextExtensions.cs	Reworks route resolution to use ASP.NET Core diagnostics metadata and original-endpoint fallback.
src/OpenTelemetry.Instrumentation.AspNetCore/CHANGELOG.md	Notes the alignment of http.route behavior with ASP.NET Core.

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