Add tests for metrics endpoint accessibility by IP

This should fail currently, since metrics are still public.
Also, I couldn't test this code locally, since I currently run VirtualBox VMs
on my dev box. I'll just see what CI says to this code.

nixos/modules/tests/private-storage.nix

@@ -308,4 +308,10 @@ in {
         $client->log($log);
         die $@;
       };
+
+      # The issuer metrics should be accessible from the monitoring network.
+      $issuer->succeed('curl --silent --insecure --fail --output /dev/null https://172.23.23.11/metrics');
+
+      # The issuer metrics should NOT be accessible from any other network.
+      $issuer->fail('curl --silent --insecure --fail --output /dev/null https://localhost/metrics');
       ''; }