.NET Framework HttpClient Instrumentation: Fix missing events on instances created before TracerProvider

[CodeBlanch]

Fixes #2324

Changes

Hook existing ServicePoints when setting up HttpWebRequest reflection hooks on .NET Framework.

TODOs

• CHANGELOG update
• Unit tests

[reyang]

LGTM with a changelog.

[codecov]

Codecov Report

Merging #2364 (88abc89) into main (da54f8c) will increase coverage by 0.04%.
The diff coverage is n/a.

❗ Current head 88abc89 differs from pull request most recent head 2132844. Consider uploading reports for the commit 2132844 to get more accurate results

@@            Coverage Diff             @@
##             main    #2364      +/-   ##
==========================================
+ Coverage   80.93%   80.97%   +0.04%     
==========================================
  Files         229      229              
  Lines        7323     7318       -5     
==========================================
- Hits         5927     5926       -1     
+ Misses       1396     1392       -4     

Impacted Files	Coverage Δ
...OpenTelemetry/Metrics/BaseExportingMetricReader.cs	71.42% <0.00%> (-1.91%)	⬇️
...Telemetry/Internal/SelfDiagnosticsEventListener.cs	96.85% <0.00%> (-0.79%)	⬇️
src/OpenTelemetry/Metrics/MetricReader.cs	72.41% <0.00%> (ø)
...Telemetry/Metrics/PeriodicExportingMetricReader.cs	0.00% <0.00%> (ø)
...lemetry/Metrics/AggregationTemporalityAttribute.cs	0.00% <0.00%> (ø)
...ter.ZPages/Implementation/ZPagesActivityTracker.cs	100.00% <0.00%> (+2.85%)	⬆️
...ZPages/Implementation/ZPagesExporterEventSource.cs	62.50% <0.00%> (+6.25%)	⬆️