Monitor by segment

### Problem Statement

I liked this Sentry blog post ([Green dashboards, red flags](https://blog.sentry.io/green-dashboards-red-flags/)), in particular this part

> Monitor them by segment. Not averages. By customer tier, by region, by device, by release. The bug that breaks your biggest customer doesn't show up in aggregate metrics.

However, I'm not sure how to achieve this at scale without manual effort creating new alerts for each of the segments. I would like to be able to be alerted if, for example, API route latency for a segment I define (e.g., tenant id) was elevated, rather than API route latency in aggregate. In theory, I could create new alerts for each tenant, but this is very tedious.

### Solution Brainstorm

Let users define a segment / group by field for an alert, then do separate alerting for each segment.

### Product Area

Alerts

Start with the Alerts product area and read the linked Sentry blog post for the motivating example. Identify how users could define a segment or group-by field and what separate alerting per segment should mean; the issue is done when the behavior and scope are specified well enough to implement and validate.

Written by the indexing model from the issue text.