[Instrumentation.Http] Nullable

[Kielek]

Towards #894

Changes

[Instrumentation.Http] Nullable except HttpWebRequestActivitySource and corresponding tests.
I think that this class deserves separate PR.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, 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

All modified and coverable lines are covered by tests ✅

Project coverage is 81.99%. Comparing base (71655ce) to head (6009308).
Report is 352 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1941      +/-   ##
==========================================
+ Coverage   73.91%   81.99%   +8.07%     
==========================================
  Files         267       16     -251     
  Lines        9615      944    -8671     
==========================================
- Hits         7107      774    -6333     
+ Misses       2508      170    -2338     

Flag	Coverage Δ
unittests-Instrumentation.GrpcNetClient	79.61% <ø> (?)
unittests-Instrumentation.Http	82.28% <100.00%> (?)

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

Files	Coverage Δ
....Instrumentation.Http/HttpClientInstrumentation.cs	100.00% <100.00%> (ø)
...tInstrumentationTracerProviderBuilderExtensions.cs	100.00% <ø> (ø)
...elemetry.Instrumentation.Http/HttpClientMetrics.cs	100.00% <100.00%> (ø)
...tion.Http/HttpClientTraceInstrumentationOptions.cs	100.00% <100.00%> (ø)
...tp/Implementation/HttpHandlerDiagnosticListener.cs	71.87% <100.00%> (ø)
...ementation/HttpHandlerMetricsDiagnosticListener.cs	88.67% <100.00%> (ø)
...plementation/HttpWebRequestActivitySource.netfx.cs	80.85% <ø> (ø)

... and 276 files with indirect coverage changes