test(golang): add Gorilla OTel semantics weblog

[marcotc]

Motivation

APMAPI-2047 extends the OpenTelemetry HTTP semantics RFC system-test coverage to a real Gorilla mux server integration.

Changes

• Add a minimal gorilla-mux Go weblog with framework-native route matching and traced net/http downstream calls.
• Reuse the router-neutral endpoint behavior introduced by test(golang): add go-restful OTel semantics weblog #7585 while leaving route registration and unmatched-request handling to Gorilla mux.
• Schedule the weblog only for the four existing OpenTelemetry HTTP semantics scenarios.
• Keep the legacy scenario-matrix parity check aligned with the restricted weblog variant.

Validation

The weblog builds and passes all forced semantic test classes against dd-trace-go commit 3f75582cb90f232ddcace4ebd2fc7a2296320851. Scenario-selection validation and relevant TEST_THE_TEST checks also pass. The full stacked tree matches the previously validated combined change, except the shared test expectation now comes from #7573.

Stack

• Base: fix(otel): correct trace metric status and endpoint coverage #7573
• Previous: test(golang): add go-restful OTel semantics weblog #7585, go-restful v3 weblog
• This PR: Gorilla mux weblog

Gotchas

The Go manifest remains unchanged because the eventual dd-trace-go release version is not known. The semantic classes therefore remain marked missing_feature until that release version can be recorded; this PR stays draft meanwhile.

Workflow

1. ⚠️ Create your PR as draft ⚠️
2. Work on you PR until the CI passes
3. Mark it as ready for review
   • Test logic is modified? -> Get a review from RFC owner.
   • Framework is modified, or non obvious usage of it -> get a review from R&P team

🚀 Once your PR is reviewed and the CI green, you can merge it!

🛟 #apm-shared-testing 🛟

Reviewer checklist

• Anything but tests/ or manifests/ is modified ? I have the approval from R&P team
• A docker base image is modified?
  • the relevant build-XXX-image label is present
• A scenario is added, removed or renamed?
  • Get a review from R&P team

[github-actions]

CODEOWNERS have been resolved as:

utils/build/docker/golang/app/_shared/otel_semantics/handlers.go        @DataDog/system-tests-reviewers
utils/build/docker/golang/app/go-restful-v3/main.go                     @DataDog/system-tests-reviewers
utils/build/docker/golang/app/gorilla-mux/main.go                       @DataDog/system-tests-reviewers
utils/build/docker/golang/go-restful-v3.Dockerfile                      @DataDog/system-tests-reviewers
utils/build/docker/golang/gorilla-mux.Dockerfile                        @DataDog/system-tests-reviewers
docs/execute/build.md                                                   @DataDog/system-tests-core
manifests/dotnet.yml                                                    @DataDog/system-tests-reviewers
manifests/golang.yml                                                    @DataDog/system-tests-reviewers
manifests/java.yml                                                      @DataDog/system-tests-reviewers
manifests/nodejs.yml                                                    @DataDog/system-tests-reviewers
manifests/php.yml                                                       @DataDog/system-tests-reviewers
manifests/python.yml                                                    @DataDog/system-tests-reviewers
manifests/ruby.yml                                                      @DataDog/system-tests-reviewers
manifests/rust.yml                                                      @DataDog/system-tests-reviewers
tests/test_otel_http_semantics.py                                       @DataDog/system-tests-core
tests/test_the_test/test_ci_orchestrator.py                             @DataDog/system-tests-core
utils/_context/_scenarios/__init__.py                                   @DataDog/system-tests-core
utils/build/docker/golang/app/go.mod                                    @DataDog/system-tests-reviewers
utils/build/docker/golang/app/go.sum                                    @DataDog/system-tests-reviewers
utils/build/docker/golang/weblog_metadata.yml                           @DataDog/system-tests-reviewers

[datadog-prod-us1-6]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 9441649 | Docs | View more details | Give us feedback!}

[link04]

LGTM nothing I could find, not sure if anyone from Go can take a look.