feat: restructure Grafana configuration, migrate alert rules to new format and add VictoriaMetrics datasource

hosts/web-arm/modules/grafana/alerting/cpu_usage.nix

@@ -0,0 +1,66 @@
+{ lib, pkgs, config, ... }:
+{
+  services.grafana.provision.alerting.rules.settings.groups = [
+    {
+      name = "CPUUsageAlerts";
+      folder = "System Alerts";
+      interval = "1m";
+
+      rules = [
+        {
+          uid = "high-cpu-usage-alert-uid";
+          title = "HighCPUUsage";
+          condition = "D"; # Condition is now D
+
+          data = [
+            # Query A: Calculate CPU usage percentage
+            {
+              refId = "A";
+              datasourceUid = "vm-datasource-uid";
+              queryType = "prometheus";
+              relativeTimeRange = { from = 60; to = 0; }; # Query over the last minute
+              model = {
+                # Calculate average CPU usage over 1m, grouped by instance and job
+                expr = ''(1 - avg by (instance, job) (rate(node_cpu_seconds_total{mode="idle"}[1m]))) * 100'';
+                legendFormat = "CPU usage on {{instance}} ({{job}})";
+                instant = false; # This is a range query
+              };
+            }
+            # Expression C: Reduce Query A to its last value, preserving labels
+            {
+              refId = "C";
+              datasourceUid = "__expr__";
+              model = {
+                type = "reduce";
+                expression = "A"; # Input is Query A
+                reducer = "last"; # Get the last value of each series in A
+              };
+            }
+            # Expression D: Apply math condition to the reduced values from C
+            {
+              refId = "D";
+              datasourceUid = "__expr__";
+              model = {
+                type = "math";
+                expression = "$C > 90"; # Alert if CPU usage from C is > 90%
+              };
+            }
+          ];
+
+          for = "5m"; # Duration the condition must be met
+          noDataState = "NoData";
+          execErrState = "Error";
+
+          annotations = {
+            summary = "High CPU usage on {{ $labels.instance }}";
+            description = ''CPU usage on {{ $labels.instance }} (job: {{ $labels.job }}) has been above 90% for more than 5 minutes. Current value: {{ if $values.C }}{{ $values.C | humanizePercentage }}{{ else }}N/A{{ end }}%.'';
+          };
+          labels = {
+            severity = "warning";
+            category = "performance";
+          };
+        }
+      ];
+    }
+  ];
+}