grafana-agent: set hostname label on all metrics

This is handy to quickly see all metrics exported by a node, without
having to mangle with the already existing `instance` label.

`hostname` is essentially a variant of `instance` but without ports.

services/monitoring/agent.nix

@@ -41,6 +41,7 @@ in
               };
             }
           ];
+          global.external_labels.hostname = config.networking.hostName;
           configs = [
             {
               name = config.networking.hostName;