[HttpClient & HttpWebRequest] Set http.request.method as per spec

[vishweshbankwar]

Towards #4994
Design discussion issue #

Changes

Fixes http.request.method attribute value on activity and metric as per spec. For details, see the linked issue

Merge requirement checklist

• CONTRIBUTING guidelines followed (nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Merging #5003 (dfb92ea) into main (5cb7a3f) will increase coverage by 0.24%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #5003      +/-   ##
==========================================
+ Coverage   83.43%   83.67%   +0.24%     
==========================================
  Files         296      296              
  Lines       12380    12377       -3     
==========================================
+ Hits        10329    10357      +28     
+ Misses       2051     2020      -31     

Flag	Coverage Δ
unittests	83.67% <100.00%> (+0.24%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...tation.AspNetCore/Implementation/HttpInListener.cs	91.79% <100.00%> (ø)
...AspNetCore/Implementation/HttpInMetricsListener.cs	87.27% <100.00%> (+0.23%)	⬆️
...tp/Implementation/HttpHandlerDiagnosticListener.cs	76.56% <100.00%> (+0.56%)	⬆️
...ementation/HttpHandlerMetricsDiagnosticListener.cs	97.77% <100.00%> (+0.10%)	⬆️
...plementation/HttpWebRequestActivitySource.netfx.cs	83.13% <100.00%> (+0.16%)	⬆️
...rumentation.Http/MeterProviderBuilderExtensions.cs	100.00% <100.00%> (ø)
...umentation.Http/TracerProviderBuilderExtensions.cs	100.00% <100.00%> (ø)
src/Shared/RequestMethodHelper.cs	100.00% <100.00%> (ø)

... and 7 files with indirect coverage changes

[CodeBlanch]

Some feedback about the helper API being confusing but LGTM