feat(otel)!: send x-langfuse-ingestion-version: 4 by default

[hassiebp]

What does this PR do?

The default OTLP exporter now sends x-langfuse-ingestion-version: 4. This opts dual-write and preview servers into direct writes, and is a no-op on events_only (the server rejects only values above 4). additional_headers can still override it. A custom span_exporter is untouched, as before; its docstring already tells users to set the header themselves.

Stack: 3 of 10. Base: #1928. Merge after #1928. It touches only span_processor.py (one line) and tests/unit/test_additional_headers_simple.py, and is stacked so the stack stays one linear order.

Type of change

• Bug fix
• New feature
• Breaking change
• Refactor
• Documentation update
• Tooling, CI, or repo maintenance

Verification

uv run --frozen ruff check .                       # All checks passed!
uv run --frozen mypy langfuse --no-error-summary   # clean
uv run --frozen pytest -q tests/unit/test_additional_headers_simple.py  # 1 failed before the change, 10 passed after
uv run --frozen pytest -n auto tests/unit          # 714 passed, 2 skipped

Checklist

• I self-reviewed the diff using code_review.md.
• I added or updated tests for behavior changes.
• I updated docs, examples, or .env.template if needed.
• I did not hand-edit generated files; if generated files changed, I used the upstream regeneration path.
• I did not commit secrets or credentials.

  

Confidence Score: 4/5

The header behavior has a non-blocking override concern, but the explicit repository import requirement must be satisfied before merging.

Summary

The PR makes the default OTLP exporter request ingestion version 4 and adds tests for the default and an exact-case override.

• The header merge needs to account for case-insensitive header names.
• The new test should follow the repository’s import rule and shut down its processor.

_{Reviews (1) · Last reviewed commit: "feat(otel)!: send x-langfuse-ingestion-v..."}

[github-actions]

@claude review

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-06T14:32:24.728572Z	95b341d	PR opened

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[claude]

Claude Code Review

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[claude]

Looks good, straightforward config change.

Reviewed the new default header addition in LangfuseSpanProcessor and confirmed the merge order ({**default_headers, **(additional_headers or {})}) still lets callers override it. Checked the accompanying test additions — they correctly assert the "4" default and the override-to-"3" case.

Extended reasoning...

The change adds one new default header key to a headers dict in span_processor.py and updates/extends a unit test file to cover it; no auth, crypto, or data-exposure logic is touched, and the existing merge pattern (default headers spread first, then additional_headers spread over them) is unchanged, so the override guarantee holds. The diff is one line of production code plus a straightforward test addition, with no CODEOWNERS concerns raised and no outstanding third-party objections in the timeline, which is why this is simple enough to approve without further human review.