Move custom metrics helpers out of EE

What does this MR do?

This is the fourth MR in a chain of MRs to move the "Custom metrics" feature into GitLab Core.

Changes in this MR:

1. Move ee/app/helpers/ee/custom_metrics_helper.rb to app/helpers/custom_metrics_helper.rb and remove the code from module EE.
2. Move the custom metric code out of ee/app/helpers/ee/environments_helper.rb and into app/helpers/environments_helper.rb.
3. Update Prometheus docs to show that the Custom metrics feature is now in GitLab Core.

Manual testing

To test the EE product, follow the steps in the issue to test create/edit/delete a custom metric.

To test the CE product, remove the ee folder (you can rename it instead) of your GDK and verify that the Prometheus integration settings page (Settings > Integrations > Prometheus) works fine. The custom metrics feature will not show up in CE until the last frontend MR (!28919 (merged)) is merged.

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