[Monitoring] Alerts should fetch data the same way as charts

[chrisronline]

With #80737, we discovered that we aren't exactly querying data the same way between our charts and alerts which is a mistake. We should find a way to share common code here and ensure the same query is executed.

There will be some small hurdles, as the charting code was never designed to return grouped data which is necessary for alerts, but it shouldn't be too hard to overcome.

[elasticmachine]

Pinging @elastic/stack-monitoring (Team:Monitoring)