[Datadog] Change Alert (Relative Delta)

This monitors the difference between the current value and the value from an hour ago. While Elastic can do "Serial Differencing," Datadog’s change() function is much more intuitive for end-users, making the translation of the "relative" versus "absolute" logic tricky.

```json
{
"name": "Sudden Drop in Active Users",
"type": "query alert",
"query": "change(avg(last_5m):sum:app.users.active{*} , 'last_1h') < -50",
"message": "Active user count dropped by more than 50 compared to an hour ago.",
"options": {
"thresholds": { "critical": -50 }
}
}
```