feat(otel): optional OpenTelemetry tracing (ADR-0025)

[muhammetsafak]

The Java mirror of babelqueue-go/otel / babelqueue.otel / @babelqueue/core/otel (ADR-0025): optional OpenTelemetry tracing in a new com.babelqueue.otel package, with io.opentelemetry:opentelemetry-api declared <optional>true</optional> — so the core stays zero-dependency at runtime for users who don't opt in.

What

• Tracing.traceIdOf / uuidOf — trace_id (a UUID) ↔ 32-hex OTel trace id (otherwise SHA-256). The already-propagated trace_id becomes the shared OTel trace across every hop/SDK — no envelope change (GR-1).
• Tracing.wrapHandler(tracer, handler) — a CONSUMER span process <urn> in the trace_id-derived trace, with messaging-semconv attributes + recordException/ERROR status. Reuses the shared Handler, so it composes with Idempotent.wrap / SchemaValidation.wrap.
• Tracing.publish(tracer, urn, data[, queue], send) — a PRODUCER span publish <urn> that stamps the active trace's id into the envelope's trace_id and writes it via a Sender.

Gate

mvn -B --no-transfer-progress verify: 59 tests pass (TracingTest: 5), JaCoCo bundle line coverage 96.8% (com.babelqueue.otel 92.9%) — ≥0.90 met, SpotBugs 0. Tests use opentelemetry-sdk-testing's InMemorySpanExporter.

Honest limit

One correlated trace per trace_id; exact cross-hop span parent-child (W3C traceparent as a transport header) is the documented follow-up.

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