public Duration idle(Instant instant) {
        Instant latestEvent = history.events().values().stream()
                                     .max(Comparator.comparing(History.Event::at))
                                     .map(History.Event::at)
                                     .get();
        return Duration.between(latestEvent, instant);
    }
Returns how long this snapshot has been idle at given instant
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/backup/Snapshot.java