Add non k8s single stat metrics dashboard template

lib/gitlab/metrics/templates/single-stat.metrics-dashboard.yml

+# Only one dashboard should be defined per file
+# More info: https://docs.gitlab.com/ee/operations/metrics/dashboards/yaml.html
+dashboard: 'Heatmap Panel Example'
+
+# For more information about the required properties of panel_groups
+# please visit: https://docs.gitlab.com/ee/operations/metrics/dashboards/yaml.html#panel-group-panel_groups-properties
+panel_groups:
+  - group: 'Server Statistics'
+    panels:
+      - title: "Memory usage"
+        # More information about heatmap panel types can be found here:
+        # https://docs.gitlab.com/ee/operations/metrics/dashboards/panel_types.html#single-stat
+        type: "single-stat"
+        metrics:
+            - query: '(node_memory_MemTotal_bytes - (node_memory_MemFree_bytes + node_memory_Buffers_bytes + node_memory_Cached_bytes)) / 1024 / 1024'
+              unit: 'MB'
+              label: "Used memory"