[otlp] Change serialization of null protobuf values

[matt-hensley]

Fixes #6138

Changes

The OTLP exporter generated protobuf payloads that Loki cannot consume when attributes contained null values.

PR extends TagWriter to include TryWriteEmptyTag method. When a null value is encountered, TryWriteEmptyTag is called in tag writer implementations instead of only returning false.

Tag writers for console, OTLP, and Zipkin exporters have new TryWriteEmptyTag implementations

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)

[rajkumar-rangaraj]

@matt-hensley Changes looks good to me, this should solve the issue. Please update the changelog.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.36%. Comparing base (0aba0cd) to head (0a0dca9).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6149   +/-   ##
=======================================
  Coverage   86.36%   86.36%           
=======================================
  Files         257      257           
  Lines       11574    11574           
=======================================
  Hits         9996     9996           
  Misses       1578     1578           

Files with missing lines	Coverage Δ
...xporter.Console/Implementation/ConsoleTagWriter.cs	11.11% <ø> (ø)
...Implementation/Serializer/ProtobufOtlpTagWriter.cs	95.45% <ø> (ø)
....Exporter.Zipkin/Implementation/ZipkinTagWriter.cs	63.63% <ø> (ø)