public FetchVector with(Dimension dimension, String value) {
        if (value == null) return makeFetchVector(merged -> merged.remove(dimension));
        return makeFetchVector(merged -> merged.put(dimension, value));
    }
Returns a new FetchVector, identical to {@code this} except for its value in {@code dimension}.
Dimension is removed if the value is null.
flags/src/main/java/com/yahoo/vespa/flags/FetchVector.java