public Function<Contexts.FeaturePostMatch, Object> valueProducerForKey(String key) {
    var getter = valueGetterForKey(key);
    return context -> getter.apply(context.parent().feature(), key);
  }
Returns a function that extracts the value for {@code key} from a {@link WithTags} instance.
planetiler-custommap/src/main/java/com/onthegomap/planetiler/custommap/TagValueProducer.java