fix: get body using request.GetBody to enable request reuse

[adomaskizogian]

fixes #8258

get http request body from GetBody if it is set, otherwise fallback to Body field.
This allows reusing the request body bytes if the same http request struct is reused on subsequent requests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.3%. Comparing base (7d8be10) to head (260a8b3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8352   +/-   ##
=====================================
  Coverage   82.3%   82.3%           
=====================================
  Files        186     186           
  Lines      13765   13772    +7     
=====================================
+ Hits       11330   11337    +7     
  Misses      2030    2030           
  Partials     405     405           

Files with missing lines	Coverage Δ
instrumentation/net/http/otelhttp/transport.go	95.6% <100.0%> (+0.2%)	⬆️

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