Make operations/metrics settings extensible

What does this MR do?

Preparation for #214370 (closed)

As metrics dashboard will have a timezone selection, the metrics settings should become more general, so they can fit more fields and configuration.

The form title and structure has been updated to prepare for more fields. Most of the changes are internal.

Before

We had a single setting as the root of the form:

• external_dashboard (form + 1 form group)

After

• metrics_settings (form)
  • external_dashboard (form group)
  • (add one more setting in the next MR)

Screenshots

Example link: http://127.0.0.1:3000/root/autodevops-deploy/-/settings/operations

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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