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, 18 insertions, 0 deletions

diff --git a/spec/lib/gitlab/graphql/loaders/batch_root_storage_statistics_loader_spec.rb b/spec/lib/gitlab/graphql/loaders/batch_root_storage_statistics_loader_spec.rb
new file mode 100644
index 00000000000..38931f7ab5e
--- /dev/null
+++ b/spec/lib/gitlab/graphql/loaders/batch_root_storage_statistics_loader_spec.rb
@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::Graphql::Loaders::BatchRootStorageStatisticsLoader do
+  describe '#find' do
+    it 'only queries once for project statistics' do
+      stats = create_list(:namespace_root_storage_statistics, 2)
+      namespace1 = stats.first.namespace
+      namespace2 = stats.last.namespace
+
+      expect do
+        described_class.new(namespace1.id).find
+        described_class.new(namespace2.id).find
+      end.not_to exceed_query_limit(1)
+    end
+  end
+end