Fix uninitialized constant SystemDashboardService in custom dashboards

app/services/metrics/dashboard/custom_metric_embed_service.rb

@@ -40,7 +40,7 @@ module Metrics
         # All custom metrics are displayed on the system dashboard.
         # Nil is acceptable as we'll default to the system dashboard.
         def valid_dashboard?(dashboard)
-          dashboard.nil? || SystemDashboardService.system_dashboard?(dashboard)
+          dashboard.nil? || ::Metrics::Dashboard::SystemDashboardService.system_dashboard?(dashboard)
         end
       end
 

changelogs/unreleased/ak-fix-undefined-value.yml

+---
+title: Fix uninitialized constant SystemDashboardService
+merge_request: 19453
+author:
+type: fixed