Make Usage ping available for admin via API

Description

Exposing the usage ping via API would enable one to build graphs and similar in an external monitoring system. I think this might be a relatively minor change in code since you already build and send the usage ping as JSON anyway.

Use cases

With the data in a TSDB, one could for instance build pretty charts displaying how and when one's GitLab instance grew significantly over time (and possibly correlate that with new projects started).

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.