Add charts showing CPU and memory usage

Add charts showing the CPU and memory usage of each instance to the
self monitoring default dashboard.

changelogs/unreleased/212882-add-cpu-mem-charts.yml

+---
+title: Add CPU, memory usage charts to self monitoring default dashboard
+merge_request: 33532
+author:
+type: changed

config/prometheus/self_monitoring_default.yml

 dashboard: 'Default dashboard'
 priority: 1
 panel_groups:
+
+- group: 'Resource usage'
+  panels:
+  - title: "Memory usage"
+    type: "line-chart"
+    y_label: "% memory used"
+    metrics:
+    - id: node_memory_usage_percentage
+      query_range: '(1 - (node_memory_MemAvailable_bytes or node_memory_MemFree_bytes + node_memory_Buffers_bytes + node_memory_Cached_bytes + node_memory_Slab_bytes) / node_memory_MemTotal_bytes) * 100'
+      unit: "%"
+      label: instance
+
+  - title: "CPU usage"
+    type: "line-chart"
+    y_label: "% CPU used"
+    metrics:
+    - id: node_cpu_usage_percentage
+      query_range: '(avg without (mode,cpu) (1 - irate(node_cpu_seconds_total{mode="idle"}[5m]))) * 100'
+      unit: "%"
+      label: instance
+
 - group: Web Service
   panels:
     - title: Web Service - Error Ratio