public Duration timeLeft() {
        if (expiresAt == Instant.MAX) return NO_TIMEOUT;
        Instant now = SystemTimer.INSTANCE.instant();
        if (now.isAfter(expiresAt)) return Duration.ZERO;
        return Duration.between(now, expiresAt);
    }
@return time left or {@link #NO_TIMEOUT} if processing has no timeout
docproc/src/main/java/com/yahoo/docproc/Processing.java