Add code owners metric to DevOps adoption page [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Adds code owner metric to GraphQL API for DevOps adoption page

Feature flag reasoning

There are some performance concerns which are being discussed here, so the feature flag was introduced as an easy switch for the metric in case of big load. We'll remove the flag when discussion finishes.

Migration output

== 20210421081724 AddCodownersDevopsAdoptionSnapshot: reverting ===============
-- remove_column(:analytics_devops_adoption_snapshots, :codeowners_used_count, :integer)
   -> 0.0054s
-- remove_column(:analytics_devops_adoption_snapshots, :total_projects_count, :integer)
   -> 0.0020s
== 20210421081724 AddCodownersDevopsAdoptionSnapshot: reverted (0.0100s) ======

== 20210421081724 AddCodownersDevopsAdoptionSnapshot: migrating ===============
-- add_column(:analytics_devops_adoption_snapshots, :total_projects_count, :integer)
   -> 0.0075s
-- add_column(:analytics_devops_adoption_snapshots, :code_owners_used_count, :integer)
   -> 0.0027s
== 20210421081724 AddCodownersDevopsAdoptionSnapshot: migrated (0.0103s) ======

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #326008 (closed)