private static Map<String, String> mergeMaps(Map<String, String> m1, Map<String, String> m2) {
    var result = new TreeMap<>(m1);
    result.putAll(m2);
    return result;
  }
Returns a copy of this instance with {@link #json}'s {@link TileArchiveMetadataJson#vectorLayers()} set to
{@code layerStats}.
planetiler-core/src/main/java/com/onthegomap/planetiler/archive/TileArchiveMetadata.java