Instrument Service Ping metric for number of Duo seats assigned

Problem

In order to measure Duo License Utilization (# of seats assigned / # of seats), we need to know how many Duo seats are actually assigned for Self-Managed and Dedicated customers.

Desired Outcome

New Service Ping metric(s) for the count of Duo seats assigned. (This is effectively the Duo version of license_billable_users.

Create four metrics:

1. Duo Pro Seats Purchased
2. Duo Pro Seats Assigned
3. Duo Enterprise Seats Purchased
4. Duo Enterprise Seats Assigned

Potential Solution

Setup instrumentation class based database metric

From #477583 (comment 2036733648):

the data on how many seats have been provisioned and used, it can be fetched via the tables: add_on_purchases.quantity and user_add_on_assignments.count for that add_on_purchase, respectively.

How to verify

Self-Managed and Dedicated installations start reporting on the new metric(s)

Further actions needed