Add `usage_count` data source and visualizations

What does this MR do and why?

As part of several steps to solving #467997 (closed), this MR adds the usage_count data source to query for a single Value Streams Dashboard count metric and return data fit for SingleStat visualization. It also creates visualization templates for each metric.

Task	Status
Create a new visualization for namespace (!183096 (merged))	✅
Add usage_count data source and visualizations	⬅️

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• #467997 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

1. Please make sure that you're on the GitLab Ultimate plan (if necessary, request a license).
2. Add a few Usage Overview single stat visualizations to the group-level Value Streams Dashboard by applying the patch below
3. Visit any group, then navigate to Analyze > Analytics dashboards in the menu
4. Select Value Streams Dashboard from the list
5. Verify that the new single stat visualizations appear atop the dashboard and are rendering correctly

Add Usage Overview Single Stats

Diff

diff --git a/ee/lib/gitlab/analytics/value_stream_dashboard/dashboards/value_streams_dashboard.yaml b/ee/lib/gitlab/analytics/value_stream_dashboard/dashboards/value_streams_dashboard.yaml
index 73b6e8e281116..276312caef6bb 100644
--- a/ee/lib/gitlab/analytics/value_stream_dashboard/dashboards/value_streams_dashboard.yaml
+++ b/ee/lib/gitlab/analytics/value_stream_dashboard/dashboards/value_streams_dashboard.yaml
@@ -3,18 +3,58 @@ title: Value Streams Dashboard
 description: Track key DevSecOps metrics throughout the development lifecycle.
 version: "2"
 panels:
-  - visualization: usage_overview
-    title: Usage overview for the %{namespaceName} %{namespaceType}
+  - visualization: groups_count
+    title: 'Groups usage count'
     gridAttributes:
       yPos: 0
       xPos: 0
-      width: 12
+      width: 4
+      height: 1
+    options: {}
+  - visualization: projects_count
+    title: 'Projects usage count'
+    gridAttributes:
+      yPos: 0
+      xPos: 4
+      width: 4
+      height: 1
+    options: {}
+  - visualization: users_count
+    title: 'Users usage count'
+    gridAttributes:
+      yPos: 0
+      xPos: 8
+      width: 4
+      height: 1
+    options: {}
+  - visualization: issues_count
+    title: 'Issues usage count'
+    gridAttributes:
+      yPos: 1
+      xPos: 0
+      width: 4
+      height: 1
+    options: {}
+  - visualization: merge_requests_count
+    title: 'Merge requests usage count'
+    gridAttributes:
+      yPos: 1
+      xPos: 4
+      width: 4
+      height: 1
+    options: {}
+  - visualization: pipelines_count
+    title: 'Pipelines usage count'
+    gridAttributes:
+      yPos: 1
+      xPos: 8
+      width: 4
       height: 1
     options: {}
   - visualization: vsd_lifecycle_metrics_table
     title: 'Lifecycle metrics for the %{namespaceName} %{namespaceType}'
     gridAttributes:
-      yPos: 1
+      yPos: 2
       xPos: 0
       width: 12
       height: 5
@@ -22,7 +62,7 @@ panels:
   - visualization: vsd_dora_metrics_table
     title: 'DORA metrics for the %{namespaceName} %{namespaceType}'
     gridAttributes:
-      yPos: 6
+      yPos: 7
       xPos: 0
       width: 12
       height: 3
@@ -30,7 +70,7 @@ panels:
   - visualization: vsd_security_metrics_table
     title: 'Security metrics for the %{namespaceName} %{namespaceType}'
     gridAttributes:
-      yPos: 9
+      yPos: 10
       xPos: 0
       width: 12
       height: 2
@@ -38,7 +78,7 @@ panels:
   - visualization: dora_performers_score
     title: DORA performers score for the %{namespaceName} %{namespaceType} (Last full calendar month)
     gridAttributes:
-      yPos: 11
+      yPos: 12
       xPos: 0
       width: 12
       height: 4
@@ -46,7 +86,7 @@ panels:
   - visualization: dora_projects_comparison
     title: Projects by DORA metric (Last full calendar month)
     gridAttributes:
-      yPos: 15
+      yPos: 16
       xPos: 0
       width: 12
       height: 4

Related to #467997 (closed)