[Publisher] Add metric descriptions in definition popup

[lilidworkin]

If you click "Set Up Metrics" in the top toolbar, then click on any metric, and then click "Define Metrics", and then hover over the metric name, you should see metric description text, as in this screenshot:

We'd like to copy this text so it also appears in the modal that pops up when you click the row. We want to insert this text underneath the first paragraph with the title "[Metric Name] Description", as in the image below:

[lilidworkin]

@nasaownsky if you have any questions about this task, you can ask Jessica @jwhrec