Logs: Add helper ctors & forceflush on OpenTelemetryLoggerProvider

[CodeBlanch]

[Goal is to pare down the amount of changes on #3305]

Changes

• Adds a couple helper ctors to make it easier to construct OpenTelemetryLoggerProviders. Primary use cases are .NET Framework where IOptions might not be in use and people wanting to create multiple providers on any runtime
• Adds a ForceFlush on OpenTelemetryLoggerProvider
• Adds a SDK event for processor ForceFlush invocation
• Nullable annotations for BaseProcessor<T>

Public APIs

namespace OpenTelemetry.Logs
{
   public class OpenTelemetryLoggerProvider
   {
      public OpenTelemetryLoggerProvider(Action<OpenTelemetryLoggerOptions> configure)) {}
      public OpenTelemetryLoggerProvider() {}
      public bool ForceFlush(int timeoutMilliseconds = Timeout.Infinite) {}
   }
}

TODOs

• Appropriate CHANGELOG.md updated for non-trivial changes
• Tests
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #3364 (44bb0b1) into main (a58c7a3) will increase coverage by 0.24%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3364      +/-   ##
==========================================
+ Coverage   85.87%   86.12%   +0.24%     
==========================================
  Files         268      268              
  Lines        9460     9475      +15     
==========================================
+ Hits         8124     8160      +36     
+ Misses       1336     1315      -21     

Impacted Files	Coverage Δ
src/OpenTelemetry/BaseProcessor.cs	66.66% <100.00%> (+6.66%)	⬆️
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	75.00% <100.00%> (+0.42%)	⬆️
.../OpenTelemetry/Logs/OpenTelemetryLoggerProvider.cs	95.16% <100.00%> (+0.71%)	⬆️
...heus/Implementation/PrometheusCollectionManager.cs	79.74% <0.00%> (-2.54%)	⬇️
...ter.ZPages/Implementation/ZPagesActivityTracker.cs	100.00% <0.00%> (+2.85%)	⬆️
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	82.35% <0.00%> (+2.94%)	⬆️
...metryProtocol/Implementation/ActivityExtensions.cs	95.60% <0.00%> (+4.39%)	⬆️
...tation/OpenTelemetryProtocolExporterEventSource.cs	85.00% <0.00%> (+10.00%)	⬆️
...xporter.OpenTelemetryProtocol/OtlpTraceExporter.cs	77.27% <0.00%> (+18.18%)	⬆️
...c/OpenTelemetry/Logs/OpenTelemetryLoggerOptions.cs	100.00% <0.00%> (+27.27%)	⬆️
... and 1 more

[reyang]

LGTM.

[CodeBlanch]

@alanwest @cijothomas Originally I added this because I needed a way to test OpenTelemetryEventSourceLogEmitter but decided to keep it. Mostly our internal logging is for failures. We should probably shore it up so we have more verbose logging for key happy-path events. What I'm thinking is that will make everything more supportable over the long term. A topic to discuss on a SIG? 😄

[cijothomas]

I think this is reasonable event. it is verbose, and should have no impact unless somone optsin to this.

[cijothomas]

@CodeBlanch could you check and fix the merge conflicts?