public static FactoryBuilder newFactoryBuilder() {
    return new FactoryBuilder();
  }
@Test void injectKindFormat() {
    B3Propagation.Factory factory = (B3Propagation.Factory) B3Propagation.newFactoryBuilder()
      .injectFormat(Span.Kind.CLIENT, Format.SINGLE)
      .build();

    assertThat(factory.injectorFactory).extracting("clientInjectorFunction")
      .isEqualTo(Format.SINGLE);
  }