[Instrumentation.AspNet] Fix analysis warnings

[martincostello]

Changes

Fix/suppress code analysis warnings in OpenTelemetry.Instrumentation.AspNet.

Contributes to #950.

[codecov]

Codecov Report

Merging #960 (12ffe63) into main (8ee613c) will decrease coverage by 0.36%.
The diff coverage is 0.00%.

❗ Current head 12ffe63 differs from pull request most recent head f3864e3. Consider uploading reports for the commit f3864e3 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #960      +/-   ##
==========================================
- Coverage   68.73%   68.37%   -0.36%     
==========================================
  Files         183      183              
  Lines        7026     7027       +1     
==========================================
- Hits         4829     4805      -24     
- Misses       2197     2222      +25     

Impacted Files	Coverage Δ
....AspNet.TelemetryHttpModule/TelemetryHttpModule.cs	5.26% <0.00%> (-0.15%)	⬇️
...mentation.AspNet/MeterProviderBuilderExtensions.cs	100.00% <ø> (ø)
...sions.PersistentStorage/PersistentStorageHelper.cs	69.69% <0.00%> (-18.19%)	⬇️
...ctions/PersistentStorageAbstractionsEventSource.cs	8.33% <0.00%> (-16.67%)	⬇️
...s.PersistentStorage.Abstractions/PersistentBlob.cs	38.09% <0.00%> (-14.29%)	⬇️
...y.Extensions.PersistentStorage/FileBlobProvider.cs	77.41% <0.00%> (-6.46%)	⬇️
...orter.Geneva/TLDExporter/UncheckedASCIIEncoding.cs	15.68% <0.00%> (-0.32%)	⬇️
....PersistentStorage/PersistentStorageEventSource.cs	19.64% <0.00%> (ø)
...Telemetry.Extensions.PersistentStorage/FileBlob.cs	84.09% <0.00%> (+8.53%)	⬆️