Clean up logging work

[reyang]

Related to #1308.

Changes

• Moved the InMemory extension to the Microsoft.Extensions.Logging namespace to make it aligned with the .NET convention.
• Extracted the custom processor/exporter example to a separate project, to keep the "get started" project really simple.
• Minor clean up.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Design discussion issue #
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1341 into master will increase coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1341      +/-   ##
==========================================
+ Coverage   78.08%   78.11%   +0.03%     
==========================================
  Files         223      224       +1     
  Lines        6369     6369              
==========================================
+ Hits         4973     4975       +2     
+ Misses       1396     1394       -2     

Impacted Files	Coverage Δ
...orter.InMemory/InMemoryExporterHelperExtensions.cs	0.00% <ø> (ø)
...rter.InMemory/InMemoryExporterLoggingExtensions.cs	0.00% <0.00%> (ø)
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	80.95% <0.00%> (+3.17%)	⬆️