public static BaggageField constant(String name, @Nullable String value) {
    return new BaggageField(name, new Constant(value));
  }
@Test void constant() {
    BaggageField constant = BaggageFields.constant("foo", "bar");
    assertThat(constant.getValue(context)).isEqualTo("bar");
    assertThat(constant.getValue(extracted)).isEqualTo("bar");

    BaggageField constantNull = BaggageFields.constant("foo", null);
    assertThat(constantNull.getValue(context)).isNull();
    assertThat(constantNull.getValue(extracted)).isNull();
  }