Add remote ip logging in path traversal middleware

🎏 Context

A few weeks ago, we enabled the path traversal middleware on gitlab.com . This middleware look at the accessed relative path (including query parameters) to detect path traversals.

When an attempt is detected, the middleware will:

• stop the request with a 400 Bad Request response.
• log the attempt.

In the past, we had cases where the logged attempts were not precise enough: they were lacking the remote_ip field. This is because the path traversal middleware is pretty early in the middleware stack. As such, the log context (which contains the remote_ip automatically) is not built, thus logs triggered by the path traversal middleware will not have that field.

We already spotted this gap previously and gitlab-org/ruby/gems/labkit-ruby#41 was opened to have a proper logging context, even in middlewares.

With this MR, we want to add the remote_ip field in the path traversal middleware without waiting for gitlab-org/ruby/gems/labkit-ruby#41. The main reason is that this field is crucial when debugging large amounts of attempts. Knowing if the bulk of the attempts come from a single IP or a small set of IPs is a useful datapoint when deciding how to apply a mitigation plan.

🤔 What does this MR do and why?

• add remote_ip field in the path traversal middleware logs.
• update the related specs.

📖 References

• https://gitlab.com/groups/gitlab-org/-/epics/13437.

🦄 Screenshots or screen recordings

No UI changes

⚗️ How to set up and validate locally

1. Enable the middleware feature flag: Feature.enable(:check_path_traversal_middleware)
2. Browse http://gdk.test:8000/foo%2F..%2Fbar, you will be greeted with the error message.

Check the logs:

$ tail -f log/application_json.log | jq
...
{
  ...
  "message": "Potential path traversal attempt detected. Feedback issue: https://gitlab.com/gitlab-org/gitlab/-/issues/520714.",
  "request_rejected": true,
  "remote_ip": "172.16.123.1",
  ...
}
...

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