I am tracking crashes for my app, and would like to set up an alert for the crash rate.

To be alerted to your Crash free rate, start by setting up a Metric Monitor for Crash Rates, here.

Choose a project, then select the Crash Free Session Rate Metric and configure your threshold and filters.

Once you've set up the above, selected an assignee for the detected issues and optionally defined your run book, select "Create New Alert" in Step 7. For your new alert you'll want to keep the trigger values and Throttling option as is to ensure you're notified when the issue is created, escalates or is resolved.
​