in_zipkin: add Zipkin v2 trace receiver

[edsiper]

Summary

Add a native Zipkin v2 JSON trace receiver backed by CTrace. The input listens on port 9411 by default, accepts POST /api/v2/spans, translates Zipkin spans into native traces, supports TLS and generic HTTP server workers, and implements checked pause/resume behavior.

The decoder validates identifiers, timestamps, durations, endpoints, tags, annotations, kinds, and status fields. Invalid batches are rejected atomically. The optional parse_string_tags setting converts compatible string values to booleans, integers, or doubles.

Compatibility

• Supports Zipkin v2 JSON.
• Zipkin v1 and protobuf payloads are not accepted.
• Supports single-listener and multi-worker ingress paths.
• No bundled library changes or new external dependencies.

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Testing

• Example configuration file for the change
• Debug log output from testing the change
• Strict Valgrind completed with no leaks or memory corruption

Verification commands:

cmake -S . -B build -DFLB_TESTS_RUNTIME=On -DFLB_TESTS_INTERNAL=On
cmake --build build -j8
tests/integration/.venv/bin/python -m pytest tests/integration/scenarios/in_zipkin/tests/test_in_zipkin.py -q
VALGRIND=1 VALGRIND_STRICT=1 tests/integration/.venv/bin/python -m pytest tests/integration/scenarios/in_zipkin/tests/test_in_zipkin.py -q

Both integration runs passed all 16 cases. The strict Valgrind run reported no leaks or memory errors.

Packaging changes are not part of this pull request.

• Run local packaging test: [N/A]
• Set ok-package-test label: [N/A]

Documentation

• Documentation required for this feature

A follow-up fluent-bit-docs change is required for the new input configuration reference.

Backporting

• Backport to latest stable release: [N/A]

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[coderabbitai]

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