Add prometheus metrics

📇 Context

The path traversal middleware is working pretty well.

As we're preparing the next steps, we noted that the observability of the middleware was lacking.

We want to have a clear view on:

• the impact of the middleware in terms of execution time.
• the amount of rejected requests.

Thus, https://gitlab.com/gitlab-org/gitlab/-/issues/498510+ was created and the very first steps is to update the middleware code to start sending (promotheus) metrics.

🤔 What does this MR do and why?

• Add promotheus metrics to the path traversal check middleware.
  • A duration histogram with custom buckets for the measured execution time.
  • A counter for the number of rejected requests.
• Update the related specs.

The path traversal check middleware is still behind a feature flag. Thus, we don't need a changelog here.

We followed https://docs.gitlab.com/ee/development/prometheus_metrics.html.

🏎 MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

✅

🦄 Screenshots or screen recordings

From the /-/metrics url:

⚙ How to set up and validate locally

In a rails console:

• Enable the general middleware feature flag : Feature.enable(:check_path_traversal_middleware)
• Enable (or not) the reject request behavior of the middleware: Feature.enable(:check_path_traversal_middleware_reject_requests). If enabled, requests with attempts are rejected with a 400 Bad Request response else all requests are accepted.

You can also browse the GitLab instance as usual. This will execute the middleware on requests without attempts.

When ready, check /-/metrics and you should see something similar to the above screen.

Related to #498510