Kafka: Headers carry metadata without touching the payload

var record = new ProducerRecord<>("orders", "order-42", payload);
record.headers().add("traceId", traceId.getBytes(UTF_8));
record.headers().add("schemaVersion", "2".getBytes(UTF_8));

// consumer
Header h = record.headers().lastHeader("traceId");
String traceId = h == null ? null : new String(h.value(), UTF_8);

The payload stays a clean domain event,
and a consumer that does not care about tracing never has to parse it.