docs: pipeline: outputs: kafka: document Protobuf serialization

[eschabell]

• Add protobuf to the format row's list of available formats.
• Add the protobuf_message row to the Configuration parameters
  table, between otlp_logs_partition_by_resource and
  queue_full_retries.
• Rescope schema_id and schema_registry_subject from Avro-only to
  Avro or Protobuf.
• Rewrite the schema_str row: it is the inline Avro alternative to a
  registry and is not supported with format: protobuf.
• Add a Protobuf support section covering the
  -DFLB_PROTOBUF_ENCODER=On build option and its Protobuf 3.12
  requirement, the mandatory schema_registry_url, recursive
  references resolution and its 64-file / 4 MiB limits,
  protobuf_message selection rules, the JSON-then-encode record
  path, Confluent wire framing, and YAML and classic examples.
• Promote "Resolve schemas from a registry" from h4 to h3 and rewrite
  its intro: the registry is enabled by either FLB_AVRO_ENCODER or
  FLB_PROTOBUF_ENCODER, not by Avro support alone.
• Promote "Kafka configuration file with raw format" to h3 so it stays
  a sibling rather than nesting under the registry section, and add
  the missing space in rawformat.
• Fix the MSK build requirements table delimiter row, which tripped
  markdownlint MD060 four times.

Signed-off-by: Eric D. Schabell eric@schabell.org

Summary by CodeRabbit

• Documentation
  • Documented Protobuf encoding support for the Kafka output plugin, including build requirements and the protobuf_message option.
  • Clarified how schema settings apply to Protobuf, including that schema_str is unsupported with this format.
  • Added details on schema registry resolution, referenced-schema limits, message selection, and startup failure messages.
  • Updated Avro and Protobuf availability guidance, and refined section organization and formatting.

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• pipeline/outputs/kafka.md

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• pipeline/outputs/kafka.md

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📝 Walkthrough

Walkthrough

The Kafka output documentation adds Protobuf format and message configuration, build requirements, schema registry behavior, encoding details, and examples. It also updates Avro support descriptions and adjusts section headings and table formatting.

Changes

Kafka Protobuf documentation

Layer / File(s)	Summary
Protobuf configuration and encoding pipeline/outputs/kafka.md	The documentation adds the protobuf format, protobuf_message, build requirements, schema graph limits, message selection rules, Confluent framing, and configuration examples.
Registry resolution and document structure pipeline/outputs/kafka.md	The documentation updates Avro and Protobuf registry resolution and support descriptions. It also promotes headings, corrects the raw-format heading text, and reformats the platform requirements table separator.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Other

Suggested reviewers: braydonk

Merge Risk: ⚪ Minimal · up to 344ab

No concrete issue was identified that would block merging the Kafka Protobuf documentation.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: documenting Protobuf serialization for the Kafka output.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[eschabell]

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[eschabell]

@patrick-stephens ready for review!