[Exporter.Instana] nullable support

[zivaninstana]

Towards #894.

[codecov]

Codecov Report

Attention: Patch coverage is 86.39053% with 23 lines in your changes missing coverage. Please review.

Project coverage is 71.42%. Comparing base (71655ce) to head (18018eb).
Report is 345 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1928      +/-   ##
==========================================
- Coverage   73.91%   71.42%   -2.49%     
==========================================
  Files         267       15     -252     
  Lines        9615      553    -9062     
==========================================
- Hits         7107      395    -6712     
+ Misses       2508      158    -2350     

Flag	Coverage Δ
unittests-Exporter.Instana	71.42% <86.39%> (?)

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

Files	Coverage Δ
...try.Exporter.Instana/Implementation/InstanaSpan.cs	100.00% <100.00%> (ø)
...Instana/Implementation/InstanaSpanTransformInfo.cs	100.00% <100.00%> (ø)
...Implementation/Processors/ActivityProcessorBase.cs	42.85% <100.00%> (-50.00%)	⬇️
...mplementation/Processors/ErrorActivityProcessor.cs	100.00% <100.00%> (ø)
...plementation/Processors/EventsActivityProcessor.cs	93.75% <100.00%> (ø)
.../OpenTelemetry.Exporter.Instana/InstanaExporter.cs	78.37% <92.85%> (ø)
...Implementation/Processors/TagsActivityProcessor.cs	91.30% <75.00%> (-8.70%)	⬇️
...er.Instana/Implementation/InstanaSpanSerializer.cs	90.37% <66.66%> (-3.43%)	⬇️
...metry.Exporter.Instana/Implementation/Transport.cs	0.00% <0.00%> (ø)
...lementation/Processors/DefaultActivityProcessor.cs	59.18% <37.50%> (+1.73%)	⬆️

... and 253 files with indirect coverage changes

[Kielek]

Public contract LTGM.
I have fixes ignoring warnings.
Other part of code I left to your judgement. Merging.

Thanks for contributing to nullability task.