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
    }
  }
}

1 files changed, 23 insertions, 0 deletions

diff --git a/lib/gitlab/graphql/loaders/batch_root_storage_statistics_loader.rb b/lib/gitlab/graphql/loaders/batch_root_storage_statistics_loader.rb
new file mode 100644
index 00000000000..a0312366d66
--- /dev/null
+++ b/lib/gitlab/graphql/loaders/batch_root_storage_statistics_loader.rb
@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Graphql
+    module Loaders
+      class BatchRootStorageStatisticsLoader
+        attr_reader :namespace_id
+
+        def initialize(namespace_id)
+          @namespace_id = namespace_id
+        end
+
+        def find
+          BatchLoader.for(namespace_id).batch do |namespace_ids, loader|
+            Namespace::RootStorageStatistics.for_namespace_ids(namespace_ids).each do |statistics|
+              loader.call(statistics.namespace_id, statistics)
+            end
+          end
+        end
+      end
+    end
+  end
+end