[sdk] Add missing XML comments and nullable decorations for public APIs

[CodeBlanch]

Changes

• Removed <NoWarn>$(NoWarn),CS0618</NoWarn> from OpenTelemetry.Api csproj. That didn't seem to introduce any new warnings.
• Removed <NoWarn>$(NoWarn),1591,CS0618</NoWarn> from OpenTelemetry csproj. This generated warnings for anything in public API missing XML docs. Added those docs.
• Added nullable annotations for everything remaining in the OpenTelemetry public API. Mostly metric files/artifacts. There is still some internal stuff missing annotations before we can turn it on for the whole SDK project, follow-up effort for that.

Merge requirement checklist

• CONTRIBUTING guidelines followed (nullable enabled, static analysis, etc.)

[codecov]

Codecov Report

Merging #4840 (ecc511c) into main (459783f) will increase coverage by 0.02%.
The diff coverage is 94.64%.

@@            Coverage Diff             @@
##             main    #4840      +/-   ##
==========================================
+ Coverage   83.91%   83.94%   +0.02%     
==========================================
  Files         293      293              
  Lines       12001    12004       +3     
==========================================
+ Hits        10071    10077       +6     
+ Misses       1930     1927       -3     

Files Changed	Coverage
src/OpenTelemetry/BaseExportProcessor.cs	ø
src/OpenTelemetry/BatchExportProcessorOptions.cs	ø
src/OpenTelemetry/CompositeProcessor.cs	ø
...OpenTelemetry/Metrics/BaseExportingMetricReader.cs	ø
...emetry/Metrics/Builder/MeterProviderBuilderBase.cs	ø
...emetry/Metrics/Exemplar/AlwaysOffExemplarFilter.cs	ø
...lemetry/Metrics/Exemplar/AlwaysOnExemplarFilter.cs	ø
...emetry/Metrics/Exemplar/SimpleExemplarReservoir.cs	0.00%
...metry/Metrics/Exemplar/TraceBasedExemplarFilter.cs	ø
src/OpenTelemetry/Metrics/ExportModesAttribute.cs	ø
... and 21 more

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[CodeBlanch]

Tag #3958