Adobe Usage metrics report to see top workspace/project users and top variables in use?

This is sort of a yes/no thing...

Right now, there isn't anything in Workspace to do this (although this has been an ask for improvement - Analytics for Analytics so to speak).

However, Admins do have access to logs

Admin > All Admin > (Data configuration & collection) > Logs

There are three types of logs:

• Admin Tools Log
• Usage & Access Log
• Report Suite Change Log

It sounds like you are mostly interested in Usage & Access Logs.

It's not the easiest to use... I know that some companies actually export data from here, and build their own report in Excel (but it really depends on how far down the rabbit hole you want to go). 

Directly in the Logs interface, you can filter for specific logins, or you can look for specific Workspaces (the Event will contain the project name), and this field uses contains logic, so you don't have to match the full value of the event (i.e. "Project Viewed: Name=My Sample Project Id=xxxxxxxxxxxxxxxxx"

Despite this, a lot of us are still advocating for better ways to analyze how users are using our Reports, Segments, Calculated Metrics, etc. Something that is easier to slice and dice the information (like a Workspace for our instance, instead of for our sites).

One quick note, for those on Adobe CJA this is easy to implement OOTB. Usage analytics has details of what's available once it's set up.