Protect metric services via firewall when exposing them

What does this MR do?

There is some confusion regarding the IP whitelist step and that it will protect all metric endpoints.

In reality, the IP whitelist will only protect the GitLab metrics endpoint since requests to this endpoint go through GitLab-Rails.

Other bundled metric services that are configured to bind on the network interface DO NOT go through the IP whitelist because these requests do not go through GitLab-Rails, rather the request goes directly to the respective metric service.

Therefore, let's add some docs to advise readers to add ingress firewall rules to protect these metric services when exposing them on the network interface.

Do note I've reordered the steps and I've added configurations examples for missing metrics (Registry, and Pgbouncer).

Related issues

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