@Test public void on_completion_should_forward_then_tag_if_exception() {

  Span span = tracing.tracer().nextSpan().start();
  Callback delegate = mock(Callback.class);
  Callback tracingCallback = TracingCallback.create(delegate, span, currentTraceContext);
  RecordMetadata md = createRecordMetadata();
  tracingCallback.onCompletion(md, error);
  verify(delegate).onCompletion(md, error);
  assertThat(spans.get(0).finishTimestamp()).isNotZero();
  assertThat(spans.get(0).error()).isEqualTo(error);
}