Allow customizing notifications for Metric Alerts

Problem Statement

I want to link a playbook to the firing notifications of a metric alert. For issue alerts, I can customize the slack messages for example with a notes field where I can do this.

With metric alerts, none of the notification destinations allow specifying any context here, and the alert doesn't have a free form description either.

This essentially makes it impossible to have a metric alert used to notify teams that are not already intimately familiar with the details of how Sentry is integrated into their organization (eg, a product team vs the infrastructure team owning the Sentry deployment)

Solution Brainstorm

Add more custom fields to the actions for metric alerts, similar to Issue Alerts. For example, for email, add a notes field here. Same for Slack:

Product Area

Alerts

Start by comparing metric alert notification actions with the issue alert Slack and email customization described in the issue. Confirm the intended custom fields and affected notification destinations with maintainers; done means metric-alert notifications can carry playbook context for the requested destinations.