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 onCompletion_shouldKeepContext_whenNotSampled() {
    Span span = tracing.tracer().nextSpan(TraceContextOrSamplingFlags.NOT_SAMPLED);

    Callback delegate =
      (metadata, exception) -> assertThat(tracing.tracer().currentSpan()).isEqualTo(span);
    Callback tracingCallback = TracingCallback.create(delegate, span, tracing.currentTraceContext());

    tracingCallback.onCompletion(null, null);
  }