@Test void linkWithErrorIsLogged() {
    List<Span> trace = List.of(
        span("a", "b", "c", Kind.CLIENT, "foo", "bar", true)
    );
    new DependencyLinker(logger).putTrace(trace).link();

    assertThat(messages).contains(
      "incrementing error link foo -> bar"
    );
  }
Spans are sometimes intermediated by an unknown type of span. Prefer the nearest server when
accounting for them.
zipkin/src/test/java/zipkin2/internal/DependencyLinkerTest.java