Add an instance variable to self monitoring dashboard

Add an instance variable that allows users to filter the resource
usage charts by the instance label.

changelogs/unreleased/212882-add-instance-variable.yml

+---
+title: Add ability to filter self monitoring resource usage charts by instance name
+merge_request: 34084
+author:
+type: changed

config/prometheus/self_monitoring_default.yml

 dashboard: 'Default dashboard'
 priority: 1
+
+templating:
+  variables:
+    instance:
+      type: 'text'
+      label: 'Instance label regex'
+      options:
+        default_value: '.+'
+
 panel_groups:
 
 - group: 'Resource usage'
@@ -9,7 +18,7 @@ panel_groups:
     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'
+      query_range: '(1 - (node_memory_MemAvailable_bytes{instance=~"{{instance}}"} or (node_memory_MemFree_bytes{instance=~"{{instance}}"} + node_memory_Buffers_bytes{instance=~"{{instance}}"} + node_memory_Cached_bytes{instance=~"{{instance}}"} + node_memory_Slab_bytes{instance=~"{{instance}}"})) / node_memory_MemTotal_bytes{instance=~"{{instance}}"}) * 100'
       unit: "%"
       label: instance
 
@@ -18,7 +27,7 @@ panel_groups:
     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'
+      query_range: '(avg without (mode,cpu) (1 - irate(node_cpu_seconds_total{mode="idle",instance=~"{{instance}}"}[5m]))) * 100'
       unit: "%"
       label: instance