instrumentation telemetry: validate session id headers

[mabdinur]

Motivation

Enable telemetry session ID header tests (DD-Session-ID, DD-Root-Session-ID, DD-Parent-Session-ID) across process forks per the Stable Service Instance Identifier RFC.

Changes

• New GET /spawn_child endpoint in default weblogs (python flask-poc, nodejs express4, ruby rails72, php, golang net-http, java spring-boot, dotnet poc). Query params: sleep, crash, fork. Fork path when supported; exec otherwise. Runtimes without fork (Java, Go, PHP, .NET) return 400 for fork=true.
• New test test_session_id_headers_across_forks in test_telemetry.py to validate session ID headers in telemetry for parent–child fork scenarios.
• Docs – endpoint spec in docs/weblog/end-to-end_weblog.md.
• Manifests – missing_feature for non-fork runtimes and weblogs without the endpoint.

Workflow

1. ⚠️ Create your PR as draft ⚠️
2. Work on your 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 is 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