Remove prometheus_custom_metrics license

Merged Reuben Pereira requested to merge 42639-remove-custom-metrics-license-check into master

What does this MR do?

As part of the effort to move the Custom metrics feature into Core, this MR:

  1. Removes the prometheus_custom_metrics license check in the metrics_controller.
  2. Removes the prometheus_custom_metrics license check in environments_helper.
  3. Removes the prometheus_custom_metrics license.
  4. Updates docs to say that the feature is now in Core.

This is the first MR in a chain of MRs. The next few MRs in the chain will move code out of the ee folder.

Issue: #42639 (closed)

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Does this MR meet the acceptance criteria?

Conformity

Availability and Testing

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
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