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

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