Expose prometheus status to monitor dashboard

What does this MR do?

As reported here the Monitor Dashboard currently displays the default placeholder message that prompts the user to install or add a cluster while Prometheus is in an updating state. This MR provides the current Prometheus state so that we can serve a more relevant message while Prometheus is updating.

Partially addresses #12453 (closed) frontend work still needed.

Screenshots

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