Add (BYO) Prometheus as cluster integration

What does this MR do?

Adds (BYO) Prometheus as cluster integration

Related: #292460 (closed)

Screenshots (strongly suggested)

Screen_Recording_2021-03-12_at_12.24.01_PM

Video showing how it works. Not shown (but documented) was installing Prometheus manually.

How to test locally

I was able to test this locally with a kind cluster

1. Install https://kind.sigs.k8s.io/
2. Create a new kind cluster.
3. In the admin section, enable local connections for webhooks and services (https://docs.gitlab.com/ee/security/webhooks.html)
4. Setup a new certificate cluster for a project.
5. Follow the newly added docs to install Prometheus, and enable Prometheus integration

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability 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