How to: $delta(cols...) - converts query results as "delta value inside interval" for Counter-like(growing only) metrics, will negative if counter reset
Example usage:

```sql
$delta(Requests) FROM requests
```

Query will be transformed into:

```sql
SELECT
    t,
    runningDifference(max_0) AS max_0_Delta
FROM
(
    SELECT
        (intDiv(toUInt32(EventTime), 60) * 60) * 1000 AS t,
        max(Requests) AS max_0
    FROM requests
    WHERE ((EventDate >= toDate(1535711819)) AND (EventDate <= toDate(1535714715)))
    AND ((EventTime >= toDateTime(1535711819)) AND (EventTime <= toDateTime(1535714715)))
    GROUP BY t
    ORDER BY t
)
```

// see [issue 455](https://github.com/Altinity/clickhouse-grafana/issues/455) for the background

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