[geneva] Nullable annotations for the metrics folder

[CodeBlanch]

Changes

• Nullable annotations for the metrics folder in GenevaExporter project.

Merge requirement checklist

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

[codecov]

Codecov Report

Attention: Patch coverage is 82.35294% with 9 lines in your changes missing coverage. Please review.

Project coverage is 58.78%. Comparing base (71655ce) to head (71ea1e2).
Report is 537 commits behind head on main.

Files with missing lines	Patch %	Lines
...eva/Metrics/OtlpProtobuf/OtlpProtobufSerializer.cs	77.77%	6 Missing ⚠️
...metry.Exporter.Geneva/Metrics/TlvMetricExporter.cs	81.25%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2218       +/-   ##
===========================================
- Coverage   73.91%   58.78%   -15.14%     
===========================================
  Files         267       45      -222     
  Lines        9615     3986     -5629     
===========================================
- Hits         7107     2343     -4764     
+ Misses       2508     1643      -865     

Flag	Coverage Δ
unittests-Exporter.Geneva	58.78% <82.35%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...xporter.Geneva/Internal/ConnectionStringBuilder.cs	95.50% <100.00%> (+2.56%)	⬆️
...emetry.Exporter.Geneva/Metrics/MetricSerializer.cs	92.30% <100.00%> (+45.19%)	⬆️
...eneva/Metrics/OtlpProtobuf/FieldNumberConstants.cs	80.00% <ø> (ø)
...Metrics/OtlpProtobuf/OtlpProtobufMetricExporter.cs	82.60% <100.00%> (ø)
...a/Metrics/OtlpProtobuf/ProtobufSerializerHelper.cs	100.00% <ø> (ø)
...er.Geneva/Metrics/OtlpProtobuf/TimestampHelpers.cs	100.00% <ø> (ø)
...s/Transport/MetricUnixDomainSocketDataTransport.cs	85.71% <ø> (ø)
...ransport/MetricWindowsEventTracingDataTransport.cs	60.00% <100.00%> (ø)
...metry.Exporter.Geneva/Metrics/TlvMetricExporter.cs	74.54% <81.25%> (ø)
...eva/Metrics/OtlpProtobuf/OtlpProtobufSerializer.cs	89.93% <77.77%> (ø)

... and 247 files with indirect coverage changes