In dev, always allow access to health endpoints from localhost

What does this MR do?

Always allow developers to access http://localhost:3000/-/metrics (and other health check endpoints) from the localhost when running in development mode.

This will allow developers to easily hook up a Prometheus instance (possibly in the GDK) in future without having to make configuration changes.

For me, it also encourages developers to explore and experiment with the metrics and understand how they work.

Corresponding improvements to Prometheus configuration in GDK: gitlab-development-kit!705 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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