public abstract TraceContext shallowCopy(TraceContext context);
@Test void shallowCopy() {
    TraceContext context = TraceContext.newBuilder().traceId(1).spanId(2).debug(true)
      .addExtra(1L).build();

    assertThat(InternalPropagation.instance.shallowCopy(context))
      .isNotSameAs(context)
      .usingRecursiveComparison()
      .isEqualTo(context);
  }