How to: $rate(cols...) - converts query results as "change rate per interval"
Example usage:

```sql
$rate(countIf(Type = 200) AS good, countIf(Type != 200) AS bad) FROM requests
```

Query will be transformed into:

```sql
SELECT
    t,
    good / runningDifference(t / 1000) AS goodRate,
    bad / runningDifference(t / 1000) AS badRate
FROM
(
    SELECT
        (intDiv(toUInt32(EventTime), 60)) * 1000 AS t,
        countIf(Type = 200) AS good,
        countIf(Type != 200) AS bad
    FROM requests
    WHERE ((EventDate >= toDate(1482796747)) AND (EventDate <= toDate(1482853383))) AND ((EventTime >= toDateTime(1482796747)) AND (EventTime <= toDateTime(1482853383)))
    GROUP BY t
    ORDER BY t
)
```

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