Backend for in-line chart designer MVC

What does this MR do?

Add a few keys to the return value of metrics_data helper method which are required for the in-line chart designer FE.

While working on the above, I noticed that the docs describe Custom metrics as a Premium feature, but the controller does not currently have any Premium license check. Which means that anyone can create custom metrics in production currently.

This MR also adds a license check to the controller to allow only Premium license holders to create/update/delete custom metrics. See comment

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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