[Datadog] "Sparse Data" Fill Logic

In Datadog, .fill(zero) or .fill(last) is handled at the query level. Elastic's bucket_script or gap_policy behaves differently. If you are comparing two sparse metrics using division, the logic often breaks during migration.

```json
{
"name": "Error Rate with Sparse Data Handling",
"type": "query alert",
"query": "avg(last_5m):default_zero(sum:api.errors{*} / sum:api.requests{*}) > 0.05",
"message": "High error rate detected. Note: default_zero used to prevent null-pointer logic. {{value}}",
"options": {
"thresholds": { "critical": 0.05 }
}
}
```