"Regular Report" Alert type

### Problem Statement

Hello, and thanks for a great product!

There are use cases where i would like to see a "regular metric report" option: a scheduled summary of key metric changes, delivered to e.g. Slack, as opposed to threshold-based alerts.

My use-cases are:

### Weekly Web-Vitals report

We have Sentry Web-Vitals alerts set up, but since these are volatile user-land metrics - we only catch major regressions this way.

We'd also like to be notified about gradual changes that don't cross alert thresholds, e.g. if something became a bit worse on average (not enough to cross alert treshold) or better (so we could consider e.g. adjusting the treshold)

Currently we cover this with manual Insights checks, but a weekly summary notification e.g. in Slack would let us quickly tell "all good" from "worth a closer look."

### Local dev-env core metrics report

We track DX metrics (rebuild speed, HMR, common errors) for our in-house SSR framework via a Sentry project. Since only a few devs are active at any given time with varying setups, our metrics are kinda volatile even at 24h interval
So currently to check "how's DX lately" we simply go a Sentry Dashboard, where we have those numbers calculated over 7D interval

A weekly report would save us from having to manually check the dashboard

### Solution Brainstorm

Generally, i would like to see something like this

In my head i see that as a separate "alert" type, with normal settings (Metric definition, Action settings, etc), but instead of "Set thresholds" it would have a "Set schedule" type of setting

### Product Area

Alerts

Start with the Alerts product area and review how existing metric definitions, threshold settings, and action settings are represented. Define the scope for a scheduled regular metric report, including its schedule and Slack delivery, before implementation; done should mean a recurring summary can be configured and delivered.

Written by the indexing model from the issue text.