Expose namespace storage statistics with GraphQL

Root namespaces have storage statistics.
This commit allows namespace owners to get those stats via GraphQL
queries like the following one

{
  namespace(fullPath: "a_namespace_path") {
    rootStorageStatistics {
      storageSize
      repositorySize
      lfsObjectsSize
      buildArtifactsSize
      packagesSize
      wikiSize
    }
  }
}

3 files changed, 8 insertions, 0 deletions

diff --git a/app/policies/group_policy.rb b/app/policies/group_policy.rb
index c686e7763bb..5d2b74b17a2 100644
--- a/app/policies/group_policy.rb
+++ b/app/policies/group_policy.rb
@@ -124,6 +124,8 @@ class GroupPolicy < BasePolicy
   rule { developer & developer_maintainer_access }.enable :create_projects
   rule { create_projects_disabled }.prevent :create_projects
 
+  rule { owner | admin }.enable :read_statistics
+
   def access_level
     return GroupMember::NO_ACCESS if @user.nil?
 
diff --git a/app/policies/namespace/root_storage_statistics_policy.rb b/app/policies/namespace/root_storage_statistics_policy.rb
new file mode 100644
index 00000000000..63fcaf20dfe
--- /dev/null
+++ b/app/policies/namespace/root_storage_statistics_policy.rb
@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+class Namespace::RootStorageStatisticsPolicy < BasePolicy
+  delegate { @subject.namespace }
+end
diff --git a/app/policies/namespace_policy.rb b/app/policies/namespace_policy.rb
index 2babcb0a2d9..937666c7e54 100644
--- a/app/policies/namespace_policy.rb
+++ b/app/policies/namespace_policy.rb
@@ -11,6 +11,7 @@ class NamespacePolicy < BasePolicy
     enable :create_projects
     enable :admin_namespace
     enable :read_namespace
+    enable :read_statistics
   end
 
   rule { personal_project & ~can_create_personal_project }.prevent :create_projects