otelhttptrace: fix header attributes lost when using sub-spans

[imorte]

When subspans are enabled (default), wroteHeaderField() writes header attributes to ct.root which points to the already ended http.getconn subspan. SetAttributes on an ended span is a silent noop, so all http.request.header.* attributes are quietly dropped.

This works correctly with WithoutSubSpans() because ct.root is set to the parent span from context, which is still active.

The fix sets ct.root to the parent span from context in both modes

[dmathieu]

Could you add a changelog entry?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.5%. Comparing base (5288efd) to head (e6ac621).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8797   +/-   ##
=====================================
  Coverage   83.4%   83.5%           
=====================================
  Files        192     192           
  Lines      15639   15636    -3     
=====================================
+ Hits       13057   13065    +8     
+ Misses      2134    2123   -11     
  Partials     448     448           

Files with missing lines	Coverage Δ
...on/net/http/httptrace/otelhttptrace/clienttrace.go	83.4% <100.0%> (-0.3%)	⬇️

... and 1 file with indirect coverage changes

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[imorte]

Could you add a changelog entry?

done

[dmathieu]

With this change, we set ct.root twice in tge same method, both with the span from context.
It seems like we could set it only once, at the start of the method?

[imorte]

With this change, we set ct.root twice in tge same method, both with the span from context. It seems like we could set it only once, at the start of the method?

fixed