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

    assertThat(new DependencyLinker().putTrace(trace).link()).containsOnly(
      DependencyLink.newBuilder().parent("client").child("server").callCount(2L).build()
    );
  }
A root span can be a client-originated trace or a server receipt which knows its peer. In these
cases, the peer is known and kind establishes the direction.
zipkin/src/test/java/zipkin2/internal/DependencyLinkerTest.java