WIP: Move custom metrics frontend code out of EE folder

What does this MR do?

This is the last MR in a chain of MRs.

This MR moves frontend code related to custom metrics out of EE.

Manual testing:

Perform the testing with the EE product as well as CE.

1. To test as EE, use a normal GitLab/GDK installation.

2. To test as CE, remove the ee folder from your Gitlab/GDK installation. You can simply rename the folder to something like ee_ignore.

   mv ee ee_ignore

Prerequisites:
Have a project with Prometheus available on your local Gitlab/GDK. You can clone https://gitlab.com/joshlambert/autodevops-deploy and use it.

Create custom metric

1. Go to project settings > Integrations > Prometheus.

2. Create a custom metric.

   You can use a simple query like up. Fill dummy data into the other fields.

Edit custom metric

1. Go to project settings > Integrations > Prometheus.

2. Edit an existing custom metric.

   Change any field and verify that the field got saved.

Check that custom metric shows up on metrics dashboard

1. Go to Operations > Metrics.
2. Check that an existing custom metric shows up.

Deleting custom metric

1. Go to project settings > Integrations > Prometheus.
2. Delete an existing custom metric.

Issue: #42639 (closed)

Screenshots

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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