evals(v4): braintrust tracedSpan OTEL branch

[miguelg719]

Why

Stack 4/6.

What

• Prepend an otel branch to tracedSpan emitting LangSmith attrs (langsmith.span.kind, input/output.value, metadata).
• Native traced()/NOOP path below is byte-identical when EVAL_TRACE_TRANSPORT is unset.

Testing

typecheck + unit + build green.

Base: #2759

---

Summary by cubic

Adds an OTEL branch to tracedSpan in the braintrust evaluator to emit LangSmith-compatible span attributes when EVAL_TRACE_TRANSPORT=otel. When unset or not "otel", behavior is unchanged; the native traced()/NOOP path remains byte-identical.

• Emits: langsmith.span.kind (llm|chain), input.value/input.mime_type (only when input is present), output.value/output.mime_type (on log), and langsmith.metadata; values are JSON-serialized and size-capped (mime_type is application/json).
• Records exceptions, sets error status on the OTEL span, and always ends it.
• To enable, set EVAL_TRACE_TRANSPORT=otel; no other changes required.

^{Written for commit 78fdf60. Summary will update on new commits.}

[changeset-bot]

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Latest commit: 78fdf60

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[cubic-dev-ai]

All reported issues were addressed across 1 file

Architecture diagram

sequenceDiagram
    participant Eval as Evaluator
    participant TS as tracedSpan()
    participant OTEL as OTel Tracer
    participant Span as OTel Span
    participant LS as LangSmith

    Note over Eval,LS: NEW: OTel transport branch for tracedSpan

    Eval->>TS: tracedSpan(fn, options)

    alt EVAL_TRACE_TRANSPORT === "otel"
        TS->>TS: resolveTraceTransport() === "otel"
        TS->>OTEL: getTracer().startActiveSpan(name, attrs)
        OTEL->>Span: Create span with LangSmith attrs
        Note over Span: langsmith.span.kind (llm|chain)<br/>input.value + input.mime_type (if input)
        
        Span->>Eval: Return span as SpanLike
        Eval->>Span: log(event)
        Alt event.output defined
            Span->>Span: Set output.value + output.mime_type
        end
        Alt event.metadata defined
            Span->>Span: Set langsmith.metadata
        end
        
        alt No exception
            Eval->>Span: fn completes
            Span->>Span: otelSpan.end()
            Span-->>TS: Return result
        else Exception thrown
            Eval->>Span: error thrown
            Span->>Span: recordException(error)
            Span->>Span: otelSpan.end()
            Span-->>TS: Re-throw error
        end
        TS-->>Eval: T (result or error)
        
    else EVAL_TRACE_TRANSPORT unset or other
        Note over TS: Byte-identical native path (unchanged)
        TS->>TS: hasBraintrustApiKey()?
    end

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