@Test public void should_add_b3_headers_to_records() {

  tracingProducer.send(producerRecord);
  List<String> headerKeys = mockProducer.history().stream()
    .flatMap(records -> Arrays.stream(records.headers().toArray()))
    .map(Header::key)
    .collect(Collectors.toList());
  assertThat(headerKeys).containsOnly("b3");
}