@Override protected String propagationField(String keyName) {
    if (keyName == null) throw new NullPointerException("keyName == null");
    Key<String> key = nameToKey.get(keyName);
    if (key == null) {
      assert false : "We currently don't support getting headers except propagation fields";
      return null;
    }
    return headers.get(key);
  }
@Test void propagationField_lastValue() {
    headers.put(b3Key, "0");
    headers.put(b3Key, "1");

    assertThat(request.propagationField("b3")).isEqualTo("1");
  }