Track web server access to Settings.pages.path

What does this MR do?

Supports #230702 (closed)

For &3980 (closed) we want to ban Puma from accessing Pages NFS. This MR lets us track each access from code under Puma as an exception in Sentry. We don't raise the exception up the call stack, we use it just to detect the Settings.pages.path method being used.

Because these exceptions are not actionable for self-managed instances, and because we don't know how much data this will send to Sentry, we use an environment variable GITLAB_PAGES_DENY_DISK_ACCESS=1 to control the tracking. It is off by default.

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