roles/logserver.nix: add initial config for loki and prometheus

roles/logserver.nix

@@ -8,5 +8,19 @@ let cfg = config.logserver;
 in {
   options.logserver = { enable = mkEnableOption "Enables logserver role"; };
 
-  config = mkIf cfg.enable { };
+  config = mkIf cfg.enable {
+    environment.systemPackages = with pkgs; [ loki prometheus ];
+
+    # Config is generated using Ansible
+    services.loki = {
+      enable = true;
+      configFile = /etc/loki/loki.yaml;
+    };
+
+    # Config is generated declaratively
+    services.prometheus = { };
+    enable = true;
+    enableReload = true;
+    checkConfig = "syntax-only";
+  };
 }