@Test public void should_add_b3_headers_when_other_headers_exist() {

  ProducerRecord<Object, String> record = new ProducerRecord<>(TEST_TOPIC, TEST_KEY, TEST_VALUE);
  record.headers().add("tx-id", "1".getBytes());
  tracingProducer.send(record);
  mockProducer.completeNext();
  MutableSpan producerSpan = spans.get(0);
  assertThat(producerSpan.kind()).isEqualTo(PRODUCER);
  assertThat(lastHeaders(mockProducer))
    .containsEntry("tx-id", "1")
    .containsEntry("b3", producerSpan.traceId() + "-" + producerSpan.id() + "-1");
}