OtelTracer.currentOtelSpan doesn't work with OTLP module

[schickling]

Summary

OtelTracer.currentOtelSpan always fails with NoSuchElementException when using the lightweight OTLP module (OtlpTracer.layer), even though Effect.currentSpan works correctly.

Current Behavior

When using the OTLP module:

• Effect.currentSpan ✅ returns the current span
• OtelTracer.currentOtelSpan ❌ always returns NoSuchElementException

Root Cause

The currentOtelSpan implementation checks for a special symbol marker:

// packages/opentelemetry/src/internal/tracer.ts:241-249
export const currentOtelSpan = Effect.flatMap(
  Effect.currentSpan,
  (span) => {
    if (OtelSpanTypeId in span) {
      return Effect.succeed((span as OtelSpan).span)
    }
    return Effect.fail(new Cause.NoSuchElementException())
  }
)

• Official OpenTelemetry approach (NodeSdk, Tracer.layer): Creates OtelSpan class instances with the OtelSpanTypeId symbol → works
• OTLP module (OtlpTracer.layer): Creates plain objects via Object.create(SpanProto) without the symbol → always fails

Workaround

// Use Effect.currentSpan instead and manually construct the span context
export const getTracingBackendUrlEffect = (span: Tracer.Span) => {
  const otelSpan = {
    spanContext: () => ({
      traceId: span.traceId,
      spanId: span.spanId,
      traceFlags: 0,
    }),
  }
  return getTracingBackendUrl(otelSpan)
}

Suggested Fix

At minimum, the JSDoc for currentOtelSpan should document this limitation:

/**
 * Get the current OpenTelemetry span.
 * 
 * **Note:** This only works when using the official OpenTelemetry API 
 * (via `Tracer.layer`, `NodeSdk.layer`, etc.). 
 * It does NOT work with the lightweight OTLP module (`OtlpTracer.layer`).
 * When using OTLP, use `Effect.currentSpan` instead.
 */

A more complete fix could provide an alternative API that works with both approaches, returning a unified interface with traceId, spanId, etc.

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