feat: add span customizers (SDK-316)

[Andrew Kent (realark)]

• support span customizers: https://github.com/braintrustdata/braintrust-spec/blob/main/skills/instrumentation-spec/references/features/span-customizers.md
• re-implement masking function with span customizer

AI Generated Description

Summary

Implements SDK-316 using the span customizer specification and the JavaScript reference.

• Add SpanCustomizer.on_span_export, SpanExportData, set_span_customizers, and auto_instrument(span_customizers=...).
• Customize native instrumentation records after lazy resolution, before merging, masking, attachments, and serialization. Manual spans, datasets, feedback, and separate OTel exporters remain outside the scope.
• Preserve identity/routing/merge fields, ordered replacements, immutable registration snapshots, fail-open continuation, and transformed-record reuse on retries.
• Mark previously unmarked Pipecat and legacy LangChain instrumentation spans; document lifecycle and fail-open redaction risks.

Verification

• Dedicated customizer suite: 10 passed.
• Isolated core suite: 863 passed, 64 skipped, 12 xfailed.
• test_types: pyright/mypy clean, 35 runtime tests passed.
• Pipecat customizer regression passed on 1.3.0 and latest (1.10.0).
• Standalone SDK smoke: incremental input/output redaction, stable span identity, manual-span exclusion.
• Changed-file pre-commit checks passed.

The core suite required clearing local Braintrust endpoint environment overrides to match recorded cassettes. It exits successfully but prints an atexit closed-output-stream warning. No cassettes were changed.

Contract note

Like JavaScript, failures are fail open. Throwing is not a safe way to block export; the API docs state this explicitly.

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-23T19:32:00.403452Z	cbcc96b	Draft marked ready

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Reviewed commit: cbcc96b492

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