VSD - Group merge request throughput should include subgroups

Enables `includeSubgroups` for the merge request throughput
group metric on the Value stream dashboard.

Changelog: fixed
EE: true

ee/app/assets/javascripts/analytics/dashboards/graphql/group_merge_requests.query.graphql

@@ -12,6 +12,7 @@ query groupMergeRequests(
       mergedAfter: $startDate
       state: $state
       labels: $labelNames
+      includeSubgroups: true
     ) {
       merge_request_throughput: count
     }