public static ScalingEvent create(ClusterResources from, ClusterResources to, long generation, Instant at) {
        return new ScalingEvent(from, to, generation, at, Optional.empty());
    }
Returns the time this event took to completion, or empty if it's not yet complete
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/applications/ScalingEvent.java