Add http.scheme tag to HttpClient instrumentation

[vishweshbankwar]

Fixes #.
part of #3373

Changes

Adding http.scheme for HttpClient tracing instrumentation. The tag already exists for metrics part and is recommended according to spec.

Please provide a brief description of the changes here.

For significant contributions please make sure you have completed the following items:

• Appropriate CHANGELOG.md updated for non-trivial changes
• Design discussion issue #
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #3464 (62b1c26) into main (95419ed) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3464      +/-   ##
==========================================
- Coverage   86.35%   86.34%   -0.02%     
==========================================
  Files         267      267              
  Lines        9633     9634       +1     
==========================================
- Hits         8319     8318       -1     
- Misses       1314     1316       +2     

Impacted Files	Coverage Δ
...tp/Implementation/HttpHandlerDiagnosticListener.cs	73.49% <100.00%> (+0.32%)	⬆️
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	79.41% <0.00%> (-2.95%)	⬇️
src/OpenTelemetry/BatchExportProcessor.cs	82.24% <0.00%> (-1.87%)	⬇️
...Telemetry/Internal/SelfDiagnosticsEventListener.cs	97.65% <0.00%> (+0.78%)	⬆️