@Override
    public Feature setAttr(String key, Object value) {
      if (value instanceof ZoomFunction || value instanceof Struct) {
        mustUnwrapValues = true;
      }
      if (value != null) {
        attrs.put(key, value);
      }
      return this;
    }
Returns the attribute to put on all output vector tile features at a zoom level.
planetiler-core/src/main/java/com/onthegomap/planetiler/FeatureCollector.java