private boolean recentlyChangedGeneration(NodeMetricSnapshot snapshot, Optional<Instant> generationChange) {
        if (generationChange.isEmpty()) return false;
        return snapshot.at().isBefore(generationChange.get().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