Add support for serializing KeyValue lists - WIP

[juliuskoval]

According to the spec, attribute values should be AnyValue. One of the possible values of AnyValue is KeyValueList, which we currently aren't able to serialize properly.

Changes

This PR adds support for serializing KeyValue lists to the OTLP exporter.

I haven't put a lot of effort into the Zipkin exporter since it is obsolete and I still need to properly implement this feature in the console exporter, so I'm only making this PR a draft.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[juliuskoval]

Question: As far as I know, the spec doesn't say anything about limits to have many nested layers there can be, but shouldn't we try to guard against self-referencing IEnumerables?

[codecov]

Codecov Report

❌ Patch coverage is 70.58824% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.80%. Comparing base (1f68cce) to head (8d385e9).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...xporter.Console/Implementation/ConsoleTagWriter.cs	0.00%	5 Missing ⚠️
....Exporter.Zipkin/Implementation/ZipkinTagWriter.cs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7015      +/-   ##
==========================================
- Coverage   88.90%   88.80%   -0.10%     
==========================================
  Files         263      263              
  Lines       12424    12458      +34     
==========================================
+ Hits        11045    11063      +18     
- Misses       1379     1395      +16     

Flag	Coverage Δ
unittests-Project-Experimental	88.47% <70.58%> (-0.12%)	⬇️
unittests-Project-Stable	88.64% <70.58%> (-0.12%)	⬇️
unittests-Solution	88.39% <70.58%> (-0.34%)	⬇️
unittests-UnstableCoreLibraries-Experimental	42.40% <ø> (-0.08%)	⬇️

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

Files with missing lines	Coverage Δ
...Implementation/Serializer/ProtobufOtlpTagWriter.cs	96.90% <100.00%> (+1.01%)	⬆️
...xporter.Console/Implementation/ConsoleTagWriter.cs	69.44% <0.00%> (-11.21%)	⬇️
....Exporter.Zipkin/Implementation/ZipkinTagWriter.cs	50.00% <0.00%> (-10.87%)	⬇️

... and 2 files with indirect coverage changes

[martincostello]

shouldn't we try to guard against self-referencing IEnumerables

I think so, otherwise it would cause a stack overflow? Could just add a recursion limit to track such occurences.