Merge branch 'monitoring-disregard-arc-memory' into 'develop'

Disregard ZFS ARC cache when monitoring free RAM

Closes #119

See merge request !331

nixos/modules/monitoring/server/grafana-dashboards/resources-overview.json

@@ -370,11 +370,31 @@
       "pluginVersion": "8.3.5",
       "targets": [
         {
-          "expr": "1 - node_memory_MemAvailable_bytes / node_memory_MemTotal_bytes",
+          "datasource": {
+            "type": "prometheus",
+            "uid": "000000001"
+          },
+          "exemplar": true,
+          "expr": "1 - node_memory_MemAvailable_bytes / node_memory_MemTotal_bytes unless node_zfs_arc_size",
+          "hide": false,
+          "instant": false,
           "interval": "",
           "intervalFactor": 4,
           "legendFormat": "{{instance}}",
-          "refId": "A"
+          "refId": "Hosts without ZFS"
+        },
+        {
+          "datasource": {
+            "type": "prometheus",
+            "uid": "000000001"
+          },
+          "exemplar": true,
+          "expr": "1 - (node_memory_MemAvailable_bytes + node_zfs_arc_size) / node_memory_MemTotal_bytes",
+          "hide": false,
+          "instant": false,
+          "interval": "",
+          "legendFormat": "{{instance}}",
+          "refId": "Hosts with ZFS"
         }
       ],
       "title": "RAM used %",