Add a scrapeConfig for mailroom-inbox metrics using GCE discovery

Split from #2680 (closed)

As part of our goal to scrape the same Prometheus Jobs that the VMs do from the Prometheus remote agents. On this particular issue will be focus in on the mtail prometheus job

Original configuration in chef for the job, (see chef role https://gitlab.com/gitlab-com/gl-infra/chef-repo/-/blob/master/roles/gprd-infra-prometheus-server-app.json)

"mailroom-inbox": {
"role_name": "gprd-infra-sd-exporter",
"exporter_port": "9117",
"scrape_interval": "120s",
"file_inventory": true
},

This issue will be about add the relevant ScrapeConfig using the GCE discovery to scrape the mailroom-inbox metrics across all VM's that aren't kubernetes.

$  knife search node 'role:gprd-infra-sd-exporter' -i
1 items found
sd-exporter-01-inf-gprd.c.gitlab-production.internal

It seems that is another exporter that runs in the sd-exporter VM

At the moment, there is not a clear place to write this scrape configuration, I will follow up with a ticket that will allow us to give home to this and other similar configurations.

You can add the relevant configuration to the helm chart https://gitlab.com/gitlab-com/gl-infra/charts/-/tree/main/gitlab/prometheus-agent