PropertyFetcher API Improvements

[CodeBlanch]

Continuing the discussion started on #1232.

Changes

Attempting to make the PropertyFetcher API slightly nicer, and a bit better on perf.

Old API:

public class PropertyFetcher

New API:

public class PropertyFetcher<T>

Usage:

        private readonly PropertyFetcher<HttpContext> stopContextFetcher = new PropertyFetcher<HttpContext>("HttpContext");
        private readonly PropertyFetcher<object> beforeActionActionDescriptorFetcher = new PropertyFetcher<object>("actionDescriptor");

Details

• When you know the type, you can provide it. That's the "HttpContext" case above. Saves a cast. In the case of value types, saves a box/unbox. A little nicer to code against.
• When you don't know the type, use object. That's the "actionDescriptor" case above. Maintains the existing behavior.

TODOs

• Update all the spots PropertyFetcher is used
• CHANGELOG.md updated for non-trivial changes
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1238 into master will increase coverage by 0.05%.
The diff coverage is 92.50%.

@@            Coverage Diff             @@
##           master    #1238      +/-   ##
==========================================
+ Coverage   79.05%   79.10%   +0.05%     
==========================================
  Files         215      215              
  Lines        6169     6170       +1     
==========================================
+ Hits         4877     4881       +4     
+ Misses       1292     1289       -3     

Impacted Files	Coverage Δ
...tp/Implementation/HttpHandlerDiagnosticListener.cs	87.67% <80.00%> (-0.17%)	⬇️
...c/OpenTelemetry/Instrumentation/PropertyFetcher.cs	95.65% <90.00%> (+8.69%)	⬆️
...umentation.AspNet/Implementation/HttpInListener.cs	94.25% <100.00%> (ø)
...tation.AspNetCore/Implementation/HttpInListener.cs	90.72% <100.00%> (+0.19%)	⬆️
...ent/Implementation/GrpcClientDiagnosticListener.cs	90.32% <100.00%> (ø)
...ient/Implementation/SqlClientDiagnosticListener.cs	78.46% <100.00%> (ø)
...us/Implementation/PrometheusExporterEventSource.cs	72.72% <0.00%> (+9.09%)	⬆️

[reyang]

LGTM.