Route Metric Alerts to incident.io

### Problem Statement

Currently, Sentry's incident.io integration only supports routing issue alerts. This limitation prevents users from leveraging incident.io incident management capabilities for metric alerts. Specifically, it's not possible to:

* Route metric alerts to incident.io for Slack channel notifications or incident creation.
* Differentiate between critical and warning states of metric alerts when notifying incident.io, leading to a lack of granular control over incident response based on severity.


### Solution Brainstorm

It would be great if the incident.io integration could be extended to support metric alerts. This could mean:

* Allowing users to configure incident.io as a destination for metric alerts within Sentry's alert rules.
* Providing options to map metric alert severity levels (critical, warning) to specific incident.io incident types or Slack channel notifications.
* Including relevant metric data (value, timestamp, threshold) within the incident.io payload.

### Product Area

Alerts

Start by locating Sentry's existing incident.io integration and the alert-rule handling for issue alerts. Trace how metric alerts expose critical and warning states, then define the integration changes needed for routing, severity mapping, and metric data in the incident.io payload. Done means metric alerts can be configured for incident.io with the requested severity-aware notifications.

Written by the indexing model from the issue text.