Add issue priority as filter in Metric alerts

### Problem Statement

Users want to create a metric alert that triggers when the number of error events reach a certain threshold. However, this limit can be rached by events from issues with low priority causing noise in the alert.

### Solution Brainstorm

Add a filter option in the metric alerts configuration to take only events from issues with a determined priority. I see plans to add priority alerts, but this use case does not seem to be listed:
https://github.com/getsentry/sentry/issues/63026
https://github.com/getsentry/sentry/issues/74468

### Product Area

Alerts