private boolean recentlyCameUp(NodeMetricSnapshot snapshot, Node node) {
        Optional<History.Event> up = node.history().event(History.Event.Type.up);
        return up.isPresent() && snapshot.at().isBefore(up.get().at().plus(warmupDuration));
    }
Returns the instant this changed to the given generation, or empty if no *change* to this generation is present
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/autoscale/NodeTimeseries.java