public static FactoryBuilder newFactoryBuilder(Propagation.Factory delegate) {
    return new FactoryBuilder(delegate);
  }
@Test void newFactory_sharingRemoteName() {
    BaggagePropagation.FactoryBuilder builder = newFactoryBuilder(B3Propagation.FACTORY);
    SingleBaggageField userName =
      SingleBaggageField.newBuilder(BaggageField.create("userName")).addKeyName("baggage").build();
    SingleBaggageField userId =
      SingleBaggageField.newBuilder(BaggageField.create("userId")).addKeyName("baggage").build();
    builder.add(userName);
    assertThatThrownBy(() -> builder.add(userId))
      .isInstanceOf(IllegalArgumentException.class)
      .hasMessage("Propagation key already in use: baggage");
  }