@Test void choosesBestTraceId() {
    List<Span> trace = List.of(
      span("7180c278b62e8f6a216a2aea45d08fc9", null, "a", Kind.SERVER, "frontend", null, false),
      span("7180c278b62e8f6a216a2aea45d08fc9", "a", "b", Kind.CLIENT, "frontend", null, false),
      span("216a2aea45d08fc9", "a", "b", Kind.SERVER, "backend", null, true)
    );

    assertThat(Trace.merge(trace)).flatExtracting(Span::traceId).containsExactly(
      "7180c278b62e8f6a216a2aea45d08fc9",
      "7180c278b62e8f6a216a2aea45d08fc9",
      "7180c278b62e8f6a216a2aea45d08fc9"
    );
  }
Some truncate an incoming trace ID to 64-bits.
zipkin/src/test/java/zipkin2/internal/TraceTest.java