@Test void singleHostSpansResultInASingleCallCount() {
    List<Span> trace = List.of(
      span("a", null, "a", Kind.CLIENT, "client", null, false),
      span("a", "a", "b", Kind.SERVER, "server", null, false)
    );

    assertThat(new DependencyLinker().putTrace(trace).link()).containsOnly(
      DependencyLink.newBuilder().parent("client").child("server").callCount(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