Update Rollups example script (#1022)

Assume we run the cron jobs on an hour basis, the following will happen:

| timeline|runs_at|last_rollup_time|curr_rollup_time|what happens|
|-------------|-------------|-------------|-------------|-------------|
|the Nth cron job runs|2021-01-05 20:00:00|2021-01-05
19:00:00|2021-01-05 20:00:00|hours included [20]|
| new http_requests are created at 2021-01-05 20:30:00, let's say
http_requests (A and B)||||
|the (N+1)th cron job |2021-01-05 21:00:00|2021-01-05
20:00:00|2021-01-05 21:00:00|hours included [21]|


Events A and B will not be added to the rollup table. To confirm:

```SQL
> select date_trunc('hour', '2021-01-05 20:30:00'::timestamp) <@ tsrange('2021-01-05 19:00:00'::timestamp, '2021-01-05 20:00:00'::timestamp, '(]');
> true
```

Events A, and B won't be added here although their `ingest_time`s are
within the range, because **they are not in the citus DB yet**, they are
created at `20:30:00` and the job is running at `20:00:00`. That's 30
minutes in between.

```SQL
> select date_trunc('hour', '2021-01-05 20:30:00'::timestamp) <@ tsrange('2021-01-05 20:00:00'::timestamp, '2021-01-05 21:00:00'::timestamp, '(]');
> false
```

Events A, and B won't be added here because their `ingest_time`s are not
within the range.


Which means that http_requests A and B will be lost.

I think the issue can be fixed if we use the timestamp itself without
any truncation in the `where` clause, so instead of using
`date_trunc('minute', ingest_time)` we just use `ingest_time`.

Author: M-Sayed

use_cases/realtime_analytics.rst

@@ -186,8 +186,7 @@ The following function wraps the rollup query up for convenience.
       SUM(response_time_msec) / COUNT(1) AS average_response_time_msec
     FROM http_request
     -- roll up only data new since last_rollup_time
-    WHERE date_trunc('minute', ingest_time) <@
-            tstzrange(last_rollup_time, curr_rollup_time, '(]')
+    WHERE ingest_time <@ tstzrange(last_rollup_time, curr_rollup_time, '(]')
     GROUP BY 1, 2;
 
     -- update the value in latest_rollup so that next time we run the