@Override public Future<RecordMetadata> send(ProducerRecord<K, V> record) {
    return this.send(record, null);
  }
@Test void send_should_tag_topic_and_key() {
    tracingProducer.send(new ProducerRecord<>(TEST_TOPIC, TEST_KEY, TEST_VALUE));
    mockProducer.completeNext();

    MutableSpan producerSpan = spans.get(0);
    assertThat(producerSpan.kind()).isEqualTo(PRODUCER);
    assertThat(producerSpan.tags())
      .containsOnly(entry("kafka.topic", TEST_TOPIC), entry("kafka.key", TEST_KEY));
  }