@Override
    public Map<String, Object> tags() {
      return tags == null ? Map.of() : tags;
    }
Partial attributes of a set of features that are known ahead of time, for example a hive-partitioned parquet input
file.
<p>
Features within this set will only add tags but not change them, and the source/layer/geometry type will be one of
the values specified in those sets. If the set is empty, the values are not known ahead of time.
planetiler-core/src/main/java/com/onthegomap/planetiler/expression/Expression.java