@Override public Future<RecordMetadata> send(ProducerRecord<K, V> record) {
    return this.send(record, null);
  }
@Test void should_add_parent_trace_when_context_injected_on_headers() {
    ProducerRecord<String, String> record = new ProducerRecord<>(TEST_TOPIC, TEST_KEY, TEST_VALUE);
    tracingProducer.injector.inject(parent, new KafkaProducerRequest(record));
    tracingProducer.send(record);
    mockProducer.completeNext();

    MutableSpan producerSpan = spans.get(0);
    assertThat(producerSpan.kind()).isEqualTo(PRODUCER);
    assertChildOf(producerSpan, parent);
    assertThat(lastHeaders(mockProducer))
      .containsEntry("b3", producerSpan.traceId() + "-" + producerSpan.id() + "-1");
  }