private FetchVector makeFetchVector(Consumer<Map<Dimension, String>> mapModifier) {
        Map<Dimension, String> mergedMap = new EnumMap<>(Dimension.class);
        mergedMap.putAll(map);
        mapModifier.accept(mergedMap);
        return new FetchVector(mergedMap);
    }
Returns a new FetchVector, identical to {@code this} except for its values in the override's dimensions.
flags/src/main/java/com/yahoo/vespa/flags/FetchVector.java