Trigger in-app messaging using custom dimensions and metrics

## What feature would you like to see?
In the In-App Messaging campaign settings, be able to add an event and some condition.

More granular control of in-app messaging, i.e. using event metrics and dimensions. For instance, if I have an event that contains a metric that is for the grade of a test (from 0-5), at the moment I'd have to set an event called grade_0, grade_1 and so on, which means thinking of this in advance when creating the analytics. If I could just hook into the grade event which contains the metric, I would be able to say in the scheduling settings "display the message when the grade is less than 3". There are number of examples I can think of like this, but hopefully this example is sufficient.

Describe the feature you would like to add, ideally proposing a specific API.

## How would you use it?
Please see above
Tell us how you'd use this feature in your app.