Add url.scheme to http.client.request.duration metric

[vishweshbankwar]

Fixes #
Design discussion issue #

Changes

Adding url.scheme to http.client.request.duration that was added recently. The attribute is required.
Spec link: https://github.com/open-telemetry/semantic-conventions/blob/main/docs/http/http-metrics.md#metric-httpclientrequestduration

Please provide a brief description of the changes here.

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 #4989 (0d4ffde) into main (cd8de07) will increase coverage by 0.18%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

❗ Current head 0d4ffde differs from pull request most recent head c9b26b1. Consider uploading reports for the commit c9b26b1 to get more accurate results

@@            Coverage Diff             @@
##             main    #4989      +/-   ##
==========================================
+ Coverage   83.38%   83.57%   +0.18%     
==========================================
  Files         295      295              
  Lines       12337    12339       +2     
==========================================
+ Hits        10287    10312      +25     
+ Misses       2050     2027      -23     

Flag	Coverage Δ
unittests	83.57% <100.00%> (+0.18%)	⬆️

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

Files	Coverage Δ
...ementation/HttpHandlerMetricsDiagnosticListener.cs	97.67% <100.00%> (+0.05%)	⬆️
...plementation/HttpWebRequestActivitySource.netfx.cs	82.96% <100.00%> (+0.03%)	⬆️

... and 5 files with indirect coverage changes