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.
• 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.
• R̶e̶n̶a̶m̶e̶s̶ ̶t̶h̶e̶ ̶̶a̶c̶t̶i̶v̶e̶_̶s̶p̶a̶n̶s̶_̶̶ ̶v̶a̶r̶i̶a̶b̶l̶e̶ ̶t̶o̶ ̶̶s̶p̶a̶n̶s̶_̶̶.̶ ̶T̶h̶i̶s̶ ̶m̶a̶p̶ ̶w̶i̶l̶l̶ ̶n̶o̶ ̶l̶o̶n̶g̶e̶r̶ ̶j̶u̶s̶t̶ ̶c̶o̶n̶t̶a̶i̶n̶ ̶a̶c̶t̶i̶v̶e̶ ̶s̶p̶a̶n̶s̶,̶ ̶i̶t̶ ̶w̶i̶l̶l̶ ̶a̶l̶s̶o̶ ̶s̶t̶o̶r̶e̶ ̶s̶p̶a̶n̶s̶ ̶e̶x̶t̶r̶a̶c̶t̶e̶d̶ ̶f̶r̶o̶m̶ ̶d̶i̶s̶t̶r̶i̶b̶u̶t̶e̶d̶ ̶t̶r̶a̶c̶i̶n̶g̶ ̶h̶e̶a̶d̶e̶r̶s̶.̶

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: 2025-01-07 17:41:42

Comparing candidate commit 306ed49 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.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.91%. Comparing base (3a8bcba) to head (306ed49).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #170   +/-   ##
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  Coverage   93.91%   93.91%           
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  Files          73       73           
  Lines        4176     4176           
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  Hits         3922     3922           
  Misses        254      254           

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