Add a dry-run mode for rack attack

What does this MR do?

This moves the config of Rack::Attack out of the initialiser and into the Gitlab module.

This way, we can avoid monkey-patching Rack::Attack but instead call the configuration on the class. As a result, we can also use the the normal prepend_if_ee strategy to extend rack attack configuration for EE.

We did this so it would be possible to implement a dry-run mode for rack attack:

This allows tracking rather than throttling certain or all configured throttles.

This can be done by setting the GITLAB_THROTTLE_DRY_RUN env variable to a comma separated list of throttles. Each of the throttles specified in that list will be tracked instead of throttled.

To disable throttling for all configured throttles, we can specify '*' in the environment variable.

When a request hits the rate limit in dry run mode, it still proceeds, but we'll emit a message in the logs

Relates to: gitlab-com/gl-infra/scalability#629 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry - We plan to remove this feature after we've enabled RackAttack for GitLab.com
• [-] Documentation (if required): RackAttack is already a working feature, the dry-run mode is not intended to become part of it
• [-] 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