docs: add under-utilized alerts

_data/rules.yml

@@ -308,7 +308,25 @@ groups:
                 query: 'node_reboot_required > 0'
                 severity: info
                 for: 4h
-
+          - name: node-exporter
+            slug: node-exporter-under-utilized
+            doc_url: https://github.com/prometheus/node_exporter
+            rules:
+              - name: Host Memory is under utilized
+                description: 'Node memory is not fully used (> 80% free). Consider reducing memory space.'
+                query: 'node_memory_MemAvailable_bytes / node_memory_MemTotal_bytes * 100 > 80'
+                severity: info
+                for: 7d
+              - name: Host Cpu is under utilized
+                description: 'CPU load is < 20%. Consider reducing the number of CPUs.'
+                query: '100 - (avg by(instance) (rate(node_cpu_seconds_total{mode="idle"}[2m])) * 100) < 20'
+                severity: info
+                for: 7d
+              - name: Host lots of free Disk space
+                description: 'Disk space is not fully used (> 80% free). Consider reducing disk space.'
+                query: '(node_filesystem_avail_bytes * 100) / node_filesystem_size_bytes > 80 and ON (instance, device, mountpoint) node_filesystem_readonly == 0'
+                severity: info
+                for: 7d
 
       - name: Docker containers
         exporters: