@Override
    public String toString() {
        return "scaling event from " + from + " to " + to + ", generation " + generation + " at " + at +
               (completion.isPresent() ? " completed " + completion.get() : "");
    }
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