public Map<String, Object> mapTags(WithTags feature) {
    if (valueRetriever.isEmpty()) {
      return feature.tags();
    } else {
      Map<String, Object> result = new HashMap<>(feature.tags());
      valueRetriever.forEach((key, retriever) -> result.put(key, retriever.apply(feature, key)));
      return result;
    }
  }
Returns a new map where every tag has been transformed (or inferred) by the registered conversions.
planetiler-custommap/src/main/java/com/onthegomap/planetiler/custommap/TagValueProducer.java