code: public static <K, M extends Message> MapWithProtoValuesSubject<?, K, M, Map<K, M>> assertThat(
        @NullableDecl Map<K, M> map) {
      return assertAbout(protos()).that(map);
    }
docs: Assert on a map with {@link Message} values.
  
  <p>This allows for the equality configurations on {@link ProtoSubject} to be applied to all
  comparison tests available on {@link MapSubject.UsingCorrespondence}.