testing(parametric): update start span parametric endpoint

[mabdinur]

Description

• Moves http_header extraction from /trace/span/start/ to a new trace/span/extract_headers endpoint. This ensures the /trace/span/start parametric endpoint aligns with the following specification.
• Renames the active_spans_ variable to spans_. This map will no longer just contain active spans, it will also store spans extracted from distributed tracing headers.
• Ensures /trace/span/finish does NOT clear the span list. Spans should only be removed from spans_ (previously active_spans_) if the trace/spans/flush is called.

Motivation

• Follow up to: DataDog/system-tests@890bccd
• After this PR is merged we can revert the changes in manifest/cpp.yaml that were introduced in parametric: Ensure start span endpoints align with the spec system-tests#3300

Additional Notes

Jira ticket: [PROJ-IDENT]

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-11-22 20:12:26

Comparing candidate commit b35ee1c in PR branch munir/update-parametric-app-start-span with baseline commit 3a8bcba in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 1 metrics, 0 unstable metrics.