Instance Statistics MVP: Add instance statistics to usage ping

Overview

In &4147 (closed), we describe an MVP for a new Instance Statistics page.

Proposal

We need to add views on this page to usage ping, so we're tracking engagement on this feature. Please add:

• A new i_analytics_* counter that measures visits on this page
• Add the new Instance Statistics page to our analytics_unique_visits_for_any_target_monthly and analytics_unique_visits_for_any_target counters, so it's included.

Reference MR: !37417 (merged)