@Override public String toString() {
    return "SpanCustomizer(" + delegate + ")";
  }
@Test void doesNotStackOverflowOnToString() {
    Span span = tracing.tracer().newTrace();
    SpanCustomizerShield shield = new SpanCustomizerShield(span);
    assertThat(shield.toString())
      .isNotEmpty()
      .isEqualTo("SpanCustomizer(RealSpan(" + span.context().traceIdString() + "/" + span.context()
        .spanIdString() + "))");
  }