ILogger integration - part 3

[reyang]

This PR follows #1308.
Based on the investigation from #1327, the actual perf diff was not significant enough to justify the engineering cost of maintaining too much duplicated logic. In this PR I'm taking a different approach - by making LogRecord a reference type, and making Exporter/Processor generic, we could reuse all the existing stuff, thus making it easier to maintain.

Changes

• Changed LogRecord from struct to class.
• Extracted exporter and processor logic, made them generic.
• Updated logging example project to show how exporter/processor would work.

Please provide a brief description of the changes here.

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 #1328 into master will increase coverage by 1.31%.
The diff coverage is 80.76%.

@@            Coverage Diff             @@
##           master    #1328      +/-   ##
==========================================
+ Coverage   76.95%   78.26%   +1.31%     
==========================================
  Files         223      220       -3     
  Lines        6435     6326     -109     
==========================================
- Hits         4952     4951       -1     
+ Misses       1483     1375     -108     

Impacted Files	Coverage Δ
...rc/OpenTelemetry.Exporter.Jaeger/JaegerExporter.cs	84.94% <ø> (ø)
....Exporter.Jaeger/JaegerExporterHelperExtensions.cs	33.33% <0.00%> (ø)
...try.Exporter.OpenTelemetryProtocol/OtlpExporter.cs	29.41% <ø> (ø)
...rc/OpenTelemetry.Exporter.ZPages/ZPagesExporter.cs	90.90% <ø> (ø)
...c/OpenTelemetry.Exporter.ZPages/ZPagesProcessor.cs	90.90% <ø> (ø)
...rc/OpenTelemetry.Exporter.Zipkin/ZipkinExporter.cs	85.89% <ø> (ø)
....Exporter.Zipkin/ZipkinExporterHelperExtensions.cs	33.33% <0.00%> (ø)
src/OpenTelemetry/BaseExporter.cs	64.28% <ø> (ø)
src/OpenTelemetry/Logs/LogRecord.cs	0.00% <ø> (ø)
...c/OpenTelemetry/Logs/OpenTelemetryLoggerOptions.cs	0.00% <0.00%> (ø)
... and 13 more

[eddynaka]

Very nice! Good that we could re-use everything!