[HttpClient & ASP.NET Core] Change metric descriptions as per spec

[vishweshbankwar]

Fixes #
Design discussion issue #

Changes

Updates the descriptions as per spec: https://github.com/open-telemetry/semantic-conventions/blob/main/docs/http/http-metrics.md

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 #4990 (3361290) into main (f7b4ae1) will decrease coverage by 0.33%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #4990      +/-   ##
==========================================
- Coverage   83.68%   83.36%   -0.33%     
==========================================
  Files         295      295              
  Lines       12339    12339              
==========================================
- Hits        10326    10286      -40     
- Misses       2013     2053      +40     

Flag	Coverage Δ
unittests	83.36% <100.00%> (-0.33%)	⬇️

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

Files	Coverage Δ
...AspNetCore/Implementation/HttpInMetricsListener.cs	86.79% <100.00%> (ø)
...ementation/HttpHandlerMetricsDiagnosticListener.cs	97.67% <100.00%> (ø)

... and 5 files with indirect coverage changes