chore(instrumentation-http): provide http.request.header.<key> at server span creation time

[vitorvasc]

Which problem is this PR solving?

The HTTP semconv spec states that http.request.header.<key> SHOULD be provided at span creation time, as it can be important for making sampling decisions. The current behavior is that instrumentation-http only captures server hequest headers after span creation inside the context.with() callback.

Fixes #6393

Short description of the changes

• Refactored headerCapture in utils.ts to return an Attributes object instead of calling span.setAttribute() directly.
• Moved server request header capture in http.ts to before _startHttpSpan, merging the captured attributes into spanOptions.attributes so they are available at span creation time.
• Updated the remaining heater capture calls to use span.setAttributes() with the returned attributes.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Existing unit tests (npm run test in @opentelemetry/instrumentation-http)
• Lint check (npm run lint)

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.50%. Comparing base (fcafab5) to head (1ee20af).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6396   +/-   ##
=======================================
  Coverage   95.50%   95.50%           
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  Files         365      365           
  Lines       11609    11609           
  Branches     2677     2677           
=======================================
  Hits        11087    11087           
  Misses        522      522           

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