Implement a metric to track usage across any AI feature

Problem

We currently don't have a metric which tracks if a user with an assigned Duo seat is actively using any Duo features.

Solution

Implement duoUsedCount which returns the number of unique users, using ani AI feature. The MVC is scopes to code suggestions and dup chat.

Additional context

This metric and Ensure that duoProAssignedUsersCount accounts f... (#485506 - closed) will be used to implement the Duo seats: Assigned and used tile on the AI Impact dashboard.