Add count of projects with CODEOWNERS file to usage ping

Problem to solve

We don't have any usage metrics for CODEOWNERS! 😱

Proposal

Add the following counts to the usage ping:

• count of merge requests with required code owners
• count of merge requests with optional code owners
• count of projects with code owners

When combined with the count of merge requests, we'll be able to understand how often they are used proportional to total number of MRs in the instance, and the number of projects they are used in relative to total projects on the instance.

Links / references