Change the schema of common_metrics.yml

What does this MR do?

This MR changes the schema of the common_metrics.yml file to the new dashboard yml format described in https://gitlab.com/gitlab-org/gitlab-ce/issues/60383

This change doesn't affect the frontend since the file is not used directly. The metrics described in common_metrics.yml are imported into the DB by db/importers/common_metrics_importer.rb. The APIs used by the frontend read the metrics from the DB (PrometheusMetric model).

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11049

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

Closes #60383 (closed)