[sdk-logs] Update LogRecord for Logger API additions

[CodeBlanch]

Relates to #4433

Changes

• Updates LogRecord for additions of Logger, SeverityText, and Severity.

Merge requirement checklist

• CONTRIBUTING guidelines followed (nullable enabled, static analysis, etc.)
• Unit tests added/updated

[codecov]

Codecov Report

Merging #4456 (ff0db7d) into main (3884bee) will decrease coverage by 0.06%.
The diff coverage is 62.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4456      +/-   ##
==========================================
- Coverage   83.41%   83.35%   -0.06%     
==========================================
  Files         313      313              
  Lines       12485    12512      +27     
==========================================
+ Hits        10414    10430      +16     
- Misses       2071     2082      +11     

Impacted Files	Coverage Δ
src/OpenTelemetry/Logs/LoggerSdk.cs	0.00% <0.00%> (ø)
src/OpenTelemetry/Logs/LogRecord.cs	62.58% <33.33%> (-2.30%)	⬇️
.../OpenTelemetry/Logs/ILogger/OpenTelemetryLogger.cs	87.75% <84.21%> (-1.00%)	⬇️

... and 3 files with indirect coverage changes

[alanwest]

Nullable or just Unspecified?

[CodeBlanch]

I feel like Nullable is more the .NET way for saying a value type is unspecified but originally I didn't have LogRecordSeverity.Unspecified defined 🤣 If you think it would be better to just default to that and drop Nullable I'm good with it.

[alanwest]

I'm ok leaving this for now. Good question though.. I think we should opt for one or the other: nullable or Unspecified. I think I'll open an issue where we can capture a punch list of small questions like this for us to circle back to before we declare stable.