Serializers spec coverage (#24017)

spec/serializers/rest/instance_serializer_spec.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe REST::InstanceSerializer do
+  let(:serialization) do
+    JSON.parse(
+      ActiveModelSerializers::SerializableResource.new(
+        record, serializer: described_class
+      ).to_json
+    )
+  end
+  let(:record) { InstancePresenter.new }
+
+  describe 'usage' do
+    it 'returns recent usage data' do
+      expect(serialization['usage']).to eq({ 'users' => { 'active_month' => 0 } })
+    end
+  end
+end