@Override public Future<RecordMetadata> send(ProducerRecord<K, V> record) {
    return this.send(record, null);
  }
@Test 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");
  }