Allow specifiying data view/index pattern in Metric Threshold Alert

[henrikno]

Describe the feature:
We need a way to specify data view or index pattern in Metric Threshold alerts.

Describe a specific use case for the feature:

This is important for two reasons:

1. Performance. Currently you can only search metrics-*, but if you're creating an alert on CPU, searching just metrics-system.cpu-* is more efficient. This even more so with remote clusters/CCS.
2. The disconnect between configuring the alert, and the index it's searching makes configuring this difficult and unintuitive. For instance this means that an alert can suddenly break because someone updated the setting, not knowing that other alerts depend on it. Also there isn't a documented API to manage this, so it might differ between environments, so it might work one cluster, but if you recreate the exact same alert on another, or export/import it, it doesn't work.

[elasticmachine]

Pinging @elastic/actionable-observability (Team: Actionable Observability)

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)

[maryam-saeidi]

We are working on a new rule called Custom threshold released under Technical Review in v8.11. We plan to deprecate the Metric threshold in favor of the new rule when it is GA.

[paulb-elastic]

@henrikno are you able to try this out with the current Custom Threshold Rule now, as we're not intending to extend the metric threshold rule any more?

[paulb-elastic]

@henrikno have you been able to confirm this?

[paulb-elastic]

@henrikno closing this as you should be able to do this in Custom Threshold rule. Please re-open or let us know if that's not the case.