@Override public Future<RecordMetadata> send(ProducerRecord<K, V> record) {
    return this.send(record, null);
  }
@Test void should_inject_child_context() {
    try (Scope scope = currentTraceContext.newScope(parent)) {
      tracingProducer.send(new ProducerRecord<>(TEST_TOPIC, TEST_KEY, TEST_VALUE));
      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");
  }