[Datadog] Cross-Metric Ratio with Dimensional Filtering

Datadog allows you to perform math across two entirely different metric names and then filter by a shared tag. In Elastic, this usually requires a complex filter_ratio in TSVB or an ES|QL query which is structured very differently from this one-liner.

```json
{
"name": "Memory Usage vs Limit Ratio",
"type": "query alert",
"query": "avg(last_5m):( sum:container.memory.usage{cluster:prod} / sum:container.memory.limit{cluster:prod} ) * 100 > 90",
"message": "Cluster {{cluster.name}} is approaching memory limits. Current: {{value}}%",
"options": {
"thresholds": { "critical": 90 }
}
}
```