feat: add span customizers (SDK-316)

[Andrew Kent (realark)]

AI Generated Description

Summary

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

• Add Braintrust::SpanCustomizer#on_span_export and ordered span_customizers: registration through init/config/exporter APIs.
• Customize detached completed OpenTelemetry SpanData after origin enrichment, before destination grouping and OTLP serialization.
• Allow mutations/replacements/rerouting while preserving trace, span, and parent IDs and isolating original application/exporter data.
• Fail the entire batch before any transmission on hook, identity, return-type, or serialization errors; no fallback to unredacted spans.
• Keep the no-customizer path unchanged and document configuration, concurrency, retry, and failure semantics.

Verification

• Customizer/exporter/origin/config/state/tracing regression selection: 76 tests, 183 assertions, zero failures/errors/skips.
• Standalone real local HTTP/OTLP smoke: redaction with preserved span identity and unchanged application data.
• Changed-file Standard lint and repository precommit passed.

Tradeoffs

Configured customization deep-copies SDK SpanData and buffers encoded destination groups before sending, providing fail-closed batch behavior. Transport retries reuse encoded bytes; an explicit export resubmission reruns hooks. Arbitrary non-marshallable state added to SpanData subclasses fails closed.

[Andrew Kent (realark)]

CI triage: this failure is reproducible on unmodified main, independently of span customizers.

• Failed job: https://github.com/braintrustdata/braintrust-sdk-ruby/actions/runs/35576909952/job/106260699045 . The ruby_llm latest appraisal resolves RubyLLM 2.0.0.
• RubyLLM 2.0.0 was released on September 18, after this repository’s September 16 green main build: https://github.com/crmne/ruby_llm/releases/tag/v2.0.0 and https://github.com/braintrustdata/braintrust-sdk-ruby/actions/runs/35139460535 .
• Exported an isolated, unmodified copy of main commit 7857ab3da6b57718543f2284cff74de81c47e8d3, installed its gemfiles/ruby_llm.gemfile, and ran bundle exec ruby -Ilib:test test/braintrust/contrib/ruby_llm/instrumentation/chat_test.rb with that Gemfile. Result: 14 runs, 9 errors reproducing the same failures.
• Breakages: removed RubyLLM::Content, removed RubyLLM::Tool.param, and OpenAI requests changing to /v1/responses while existing cassettes cover /v1/chat/completions.

The PR CI job completed its preceding appraisal suites, including customizer cases, before stopping at this unrelated RubyLLM upgrade. Other matrix jobs were fail-fast cancelled. No test skips, weakened cassette matchers, or dependency downgrades were introduced to hide this failure. RubyLLM 2.0 compatibility needs a separate integration update; this span-customizer PR is still blocked by that baseline issue.