static Callback create(@Nullable Callback delegate, Span span, CurrentTraceContext current) {
    if (delegate == null) return new FinishSpan(span);
    return new DelegateAndFinishSpan(delegate, span, current);
  }
@Test 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);
  }