Add Service Template to usage ping

Proposal

Add the total number of Service Templates to the counts dictionary of the usage ping payload. That is all. 😉

The new counter will follow the pattern templates_#{service_name}_active, this is similar to what we do for the instance-level integrations, see #204802 (closed).

Documentation

1. A note should be added to the usage ping documentation noting this change.

Follow-up

1. This data should be added to the Periscope dashboard here: https://app.periscopedata.com/app/gitlab/577690/Project-Service-Usage-Dashboard