feat: implement centralized alerting with vmalert and Grafana, add alert rules for CPU, disk, inode, RAM usage, and host status

hosts/web-arm/modules/vmalert/rules/inode_usage.nix

@@ -0,0 +1,27 @@
+{ lib, pkgs, config, ... }: # Standard module arguments
+{
+  services.vmalert.rules.groups = [
+    {
+      name = "InodeUsageAlerts";
+      rules = [
+        {
+          alert = "HighInodeUsage";
+          expr = ''
+            (
+              node_filesystem_files{fstype!~"tmpfs|rootfs",mountpoint!=""} - node_filesystem_files_free{fstype!~"tmpfs|rootfs",mountpoint!=""}
+            ) / node_filesystem_files{fstype!~"tmpfs|rootfs",mountpoint!=""} * 100 > 80
+          '';
+          for = "30m";
+          labels = {
+            severity = "warning";
+            category = "capacity";
+          };
+          annotations = {
+            summary = "High inode usage on {{ $labels.instance }} at {{ $labels.mountpoint }}";
+            description = "Inode usage on {{ $labels.instance }} for mount point {{ $labels.mountpoint }} (fstype: {{ $labels.fstype }}) has been above 80% for more than 30 minutes. Current value: {{ $value | printf \"%.2f\" }}%.";
+          };
+        }
+      ];
+    }
+  ];
+}