Allow Renaming of the "Instances" Metric

[#1]

It would be snazzy if Admins could override the name of the "Instances" metric for various reports.

Examples:

- for On-Site Search Keywords reports, we'd rename it to "Searches"

- for Product Type reports, we'd rename it to "Product Views"

- for On-Site Campaign reports, we'd rename it to "Clicks"

We DO have success events set  up to augment most of these examples, but it would be nice to be able to define that wacky "Instances" up front for end users -- instead of explaining it to them one by one as they notice it in the "Add Metrics" list.