Convert additional purchased storage size in api

Converts additional_purchased_storage_size returned from megabytes to
bytes as expected.  As it turns out the unit of measure stored in the
DB was MB so this value must be converted to be consistent with other
endpoint values (e.g. total_repository_size,
actual_repository_size_limit)

ee/app/graphql/ee/types/namespace_type.rb

@@ -9,7 +9,8 @@ module EE
         field :additional_purchased_storage_size,
               GraphQL::FLOAT_TYPE,
               null: true,
-              description: 'Additional storage purchased for the root namespace in bytes'
+              description: 'Additional storage purchased for the root namespace in bytes',
+              resolve: -> (obj, _args, _ctx) { obj.additional_purchased_storage_size.megabytes }
 
         field :total_repository_size_excess,
               GraphQL::FLOAT_TYPE,

ee/spec/graphql/ee/types/namespace_type_spec.rb

@@ -18,4 +18,31 @@ RSpec.describe GitlabSchema.types['Namespace'] do
 
     expect(described_class).to include_graphql_fields(*expected_fields)
   end
+
+  describe 'Customized fields' do
+    let_it_be(:user) { create(:user) }
+    let_it_be(:group) { create(:group, additional_purchased_storage_size: 100, repository_size_limit: 10_240) }
+    let_it_be(:group_member) { create(:group_member, group: group, user: user) }
+    let_it_be(:query) do
+      %(
+        query {
+          namespace(fullPath: "#{group.full_path}") {
+            additionalPurchasedStorageSize
+            containsLockedProjects
+            actualRepositorySizeLimit
+          }
+        }
+      )
+    end
+
+    subject { GitlabSchema.execute(query, context: { current_user: user }).as_json }
+
+    it "returns the expected values for customized fields defined in NamespaceType" do
+      namespace = subject.dig('data', 'namespace')
+
+      expect(namespace['additionalPurchasedStorageSize']).to eq(100.megabytes)
+      expect(namespace['containsLockedProjects']).to eq(false)
+      expect(namespace['actualRepositorySizeLimit']).to eq(10_240)
+    end
+  end
 end