Explicitly handle class constant references

What does this MR do?

1. Updates ReactiveCachingWorker to use String#constantize instead of Kernel#const_get.

   Why: After a reload!, calling Kernel.const_get("Clusters::Applications::Prometheus") returns the module Prometheus instead of the class Clusters::Applications::Prometheus (likely due to lack of support for nesting in Kernel#const_get).

2. Updates every reference to the Metrics class within lib/gitlab/metrics is explicitly called referenced as ::Gitlab::Metrics

   Why: Rails can't find which Metrics class is required after reload. Plus some consistency is nice.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/54320

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer