Node VPN alert for Subscription

[DavidePrincipi]

Raise a my.nethserver.com / my.nethesis.it alert if a worker node Wireguard VPN link is down.

[gsanchietti]

The new monitor system uses node_exporter metrics to generate the alerts.
Before proceeding with QA of this issue, please make sure to have installed the latest version from #7162

An alert is raised when Prometheus can't reach a target node.

Test case

• Register the cluster with a valid subscription token
• Disable node_exporter on the worker node: systemctl stop node_exporter
• Verify no alert is sent, inside the logs you should see something like:

  Feb 25 08:48:53 rl1.leader.cluster0.gs.nethserver.net alert-proxy[137405]: ALERT CRITICAL node:offline:1
  

• Verify the alert is received by my.nethesis.it
• Restart node_exporter on the worker_node: systemctl start node_exporter
• Verify the alert is resolved inside my.nethesis.it

[DavidePrincipi]

In testing core 3.5.0-dev.5