Add support for KVList values in TagWriter

[slang25]

Fixes #
Design discussion issue #

Changes

Please provide a brief description of the changes here.

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)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: slang25 / name: Stuart Lang (0e45fee, ae09b80, d34d7bd)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.88%. Comparing base (866e218) to head (ae09b80).
⚠️ Report is 35 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6812      +/-   ##
==========================================
- Coverage   86.88%   86.88%   -0.01%     
==========================================
  Files         262      262              
  Lines       12350    12349       -1     
==========================================
- Hits        10730    10729       -1     
  Misses       1620     1620              

Flag	Coverage Δ
unittests-UnstableCoreLibraries-Experimental	86.07% <ø> (ø)

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

Files with missing lines	Coverage Δ
...xporter.Console/Implementation/ConsoleTagWriter.cs	10.00% <ø> (ø)
...Implementation/Serializer/ProtobufOtlpTagWriter.cs	95.89% <ø> (-0.06%)	⬇️
....Exporter.Zipkin/Implementation/ZipkinTagWriter.cs	60.86% <ø> (ø)

[rajkumar-rangaraj]

Thanks for the contribution!

This PR modifies the attribute serialization hot path (TryWriteTag), which executes for every attribute on every telemetry item. Could you run the existing OpenTelemetry.Exporter.OpenTelemetryProtocol.Benchmarks and share before/after results?

What's the motivation?

I don't see a linked issue. I'd recommend creating an issue first to drive clarity on:

1. Which backends actually benefit from kvlist over JSON string?
2. What's the OTel spec guidance on when to use kvlist vs string?

This will help us align on whether this is the right approach before investing more time in the implementation.

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