WIP: Adds cluster metrics importer

What does this MR do?

This MR adds a cluster metrics importer that is very similar to the common metrics importer. This is the first step in the process of moving cluster metrics to the prometheus endpoint and is a part of https://gitlab.com/gitlab-org/gitlab-ce/issues/63044.

Note: this is dependent on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29572 which will add a new field metric_type to PrometheusMetrics in order to differentiate between "environment" and "cluster" metrics.

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