Add last_activity_on to BillableMember entity

doc/api/members.md

@@ -284,6 +284,7 @@ Example response:
     "state": "active",
     "avatar_url": "https://www.gravatar.com/avatar/c2525a7f58ae3776070e44c106c48e15?s=80&d=identicon",
     "web_url": "http://192.168.1.8:3000/root",
+    "last_activity_on": "2021-01-27"
   },
   {
     "id": 2,
@@ -292,7 +293,8 @@ Example response:
     "state": "active",
     "avatar_url": "https://www.gravatar.com/avatar/c2525a7f58ae3776070e44c106c48e15?s=80&d=identicon",
     "web_url": "http://192.168.1.8:3000/root",
-    "email": "john@example.com"
+    "email": "john@example.com",
+    "last_activity_on": "2021-01-25"
   },
   {
     "id": 3,
@@ -300,7 +302,8 @@ Example response:
     "name": "Foo bar",
     "state": "active",
     "avatar_url": "https://www.gravatar.com/avatar/c2525a7f58ae3776070e44c106c48e15?s=80&d=identicon",
-    "web_url": "http://192.168.1.8:3000/root"
+    "web_url": "http://192.168.1.8:3000/root",
+    "last_activity_on": "2021-01-20"
   }
 ]
 ```

ee/changelogs/unreleased/vij-add-last-activity.yml

+---
+title: Add last_activity_on attribute to Billable Members API
+merge_request: 52666
+author:
+type: changed

ee/lib/ee/api/entities/billable_member.rb

@@ -5,6 +5,7 @@ module EE
     module Entities
       class BillableMember < ::API::Entities::UserBasic
         expose :public_email, as: :email
+        expose :last_activity_on
       end
     end
   end

ee/spec/lib/ee/api/entities/billable_member_spec.rb

@@ -3,10 +3,16 @@
 require 'spec_helper'
 
 RSpec.describe ::EE::API::Entities::BillableMember do
-  let(:member) { build(:user, public_email: public_email, email: 'private@email.com') }
+  let(:last_activity_on) { Date.today }
+  let(:public_email) { nil }
+  let(:member) { build(:user, public_email: public_email, email: 'private@email.com', last_activity_on: last_activity_on) }
 
   subject(:entity_representation) { described_class.new(member).as_json }
 
+  it 'returns the last_activity_on attribute' do
+    expect(entity_representation[:last_activity_on]).to eq last_activity_on
+  end
+
   context 'when the user has a public_email assigned' do
     let(:public_email) { 'public@email.com' }