@Test public void on_completion_should_tag_if_exception() {

  Span span = tracing.tracer().nextSpan().start();
  Callback tracingCallback = TracingCallback.create(null, span, currentTraceContext);
  tracingCallback.onCompletion(null, error);
  assertThat(spans.get(0).finishTimestamp()).isNotZero();
  assertThat(spans.get(0).error()).isEqualTo(error);
}