Add simple disk pressure alert

nix/systems/x86_64-linux/monitoring.kaareskovgaard.net/alerts/instance.yaml

@@ -0,0 +1,18 @@
+- name: Instance
+  rules:
+    - alert: DiskPressure
+      expr: >
+        (
+          node_filesystem_avail_bytes{fstype!="ramfs",job="integrations/node_exporter"} /
+          node_filesystem_size_bytes{fstype!="ramfs",job="integrations/node_exporter"}
+        ) < 0.2
+        and
+        (
+          node_filesystem_avail_bytes{fstype!="ramfs",job="integrations/node_exporter"} /
+          1024 / 1024 / 1024
+        ) < 20
+      for: 10m
+      labels:
+        severity: warning
+      annotations:
+        summary: "Disk pressure on {{ $labels.mountpoint }} on {{ $labels.instance }}"

nix/systems/x86_64-linux/monitoring.kaareskovgaard.net/default.nix

@@ -122,6 +122,7 @@ in
       ''
         groups:
         ${builtins.readFile ./alerts/http.yaml}
+        ${builtins.readFile ./alerts/instance.yaml}
         ${builtins.readFile ./alerts/postfix.yaml}
         ${builtins.readFile ./alerts/postgres.yaml}
         ${builtins.readFile ./alerts/systemd.yaml}