Adds cluster health group to PrometheusMetric

What does this MR do?

Based on discussion here https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29572#note_181268932, this adds the Cluster Health group to PrometheusMetric. This will be used to differentiate "environment" and "cluster" metrics.

No current metrics will get assigned to the new group created in this MR. This MR is made in preparation for later work that will import Cluster Health metrics to the database and in turn, get assigned the new group created in this MR.

EE backport: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14193

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 #63044 (moved)