Add unique to Generator for Internal Events

Problem

We want to continue progressing toward our event system vision outlined in the prototypical documentation and our objective to deprecate the old system. As part of this we want to make generating new metrics as easy as possible. We need to make sure the metric generator for internal events stays in step with the changes we do to the new Internal Event tracking system so that it actually stays usable.

Desired Outcome

The generator for internal events is up to date with the changes in Add unique_on to Internal Events Tracking (#409993 - closed). As part of this the generator should be documented as a "quick start" to create a new event-based metric.