Clarification on Using Fabric Metrics App for Multiple Capacities and Subscriptions

This depends on your tenant setup.  The capacity metrics app will show you all the capacities in your tenant that you have access to. There is usually no need to use different capacity metrics apps inside a tenant.  The story is different across tenants.

Note: They keep moving the goal posts (ie changing the semantic model)  frequently so it is worth updating to the newer versions occasionally.