refactor test exporters, use instead DelegatingTestExporter

[TimothyMothra]

Pulling changes out of #3476.

Here I'm refactoring DelegatingTestExporter to enable additional test scenarios.
Currently DelegatingTestExporter is coded for a specific scenario, recording ExportResults and performing an optional configured Action.
This change makes the entire Export configurable with OnExportFunc.

The other PR will build on this by adding additional configurable methods.

Changes

• refactor DelegatingTestExporter to be more flexible for additional test scenarios.
  (I'll expand on this in my other PR).

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 #3486 (269189a) into main (5a327df) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3486      +/-   ##
==========================================
+ Coverage   86.73%   86.76%   +0.03%     
==========================================
  Files         275      275              
  Lines        9963     9963              
==========================================
+ Hits         8641     8644       +3     
+ Misses       1322     1319       -3     

Impacted Files	Coverage Δ
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	76.47% <0.00%> (-5.89%)	⬇️
...metryProtocol/Implementation/ActivityExtensions.cs	95.60% <0.00%> (+1.09%)	⬆️
src/OpenTelemetry/Logs/OpenTelemetryLogger.cs	91.11% <0.00%> (+2.22%)	⬆️
...ter.ZPages/Implementation/ZPagesActivityTracker.cs	100.00% <0.00%> (+2.85%)	⬆️
...ZPages/Implementation/ZPagesExporterEventSource.cs	62.50% <0.00%> (+6.25%)	⬆️

[cijothomas]

LGTM