@Test void singleHostSpansResultInASingleErrorCount_sameId() {
    List<Span> trace = List.of(
      span("a", null, "a", Kind.CLIENT, "client", null, true),
      span("a", null, "a", Kind.SERVER, "server", null, true)
        .toBuilder().shared(true).build()
    );

    assertThat(new DependencyLinker().putTrace(trace).link()).containsOnly(
      DependencyLink.newBuilder()
        .parent("client")
        .child("server")
        .callCount(1L)
        .errorCount(1L)
        .build()
    );
  }
Spans don't always include both the client and server service. When you know the kind, you can
link these without duplicating call count.
zipkin/src/test/java/zipkin2/internal/DependencyLinkerTest.java