Return max_seats_used in namespaces list API

If a namespace has a GitLab subscription, return the subscription's
max_seats_used value.

ee/changelogs/unreleased/rp-namespace-list-max-seats-used.yml

+---
+title: Return max_seats_used in namespaces list API
+merge_request: 42644
+author:
+type: changed

ee/lib/ee/api/entities/namespace.rb

@@ -9,6 +9,7 @@ module EE
         prepended do
           can_update_limits = ->(namespace, opts) { ::Ability.allowed?(opts[:current_user], :update_subscription_limit, namespace) }
           can_admin_namespace = ->(namespace, opts) { ::Ability.allowed?(opts[:current_user], :admin_namespace, namespace) }
+          has_gitlab_subscription = ->(namespace) { namespace.gitlab_subscription.present? }
 
           expose :shared_runners_minutes_limit, if: can_update_limits
           expose :extra_shared_runners_minutes_limit, if: can_update_limits
@@ -17,6 +18,9 @@ module EE
           expose :billable_members_count do |namespace, options|
             namespace.billable_members_count(options[:requested_hosted_plan])
           end
+          expose :max_seats_used, if: has_gitlab_subscription do |namespace, _|
+            namespace.gitlab_subscription.max_seats_used
+          end
           expose :plan, if: can_admin_namespace do |namespace, _|
             namespace.actual_plan_name
           end

ee/spec/requests/api/namespaces_spec.rb

@@ -5,9 +5,10 @@ require 'spec_helper'
 RSpec.describe API::Namespaces do
   let(:admin) { create(:admin) }
   let(:user) { create(:user) }
-  let!(:group1) { create(:group, name: 'test.test-group.2') }
-  let!(:group2) { create(:group, :nested) }
-  let!(:gold_plan) { create(:gold_plan) }
+
+  let_it_be(:group1, reload: true) { create(:group, name: 'test.test-group.2') }
+  let_it_be(:group2) { create(:group, :nested) }
+  let_it_be(:gold_plan) { create(:gold_plan) }
 
   describe "GET /namespaces" do
     context "when authenticated as admin" do
@@ -114,6 +115,30 @@ RSpec.describe API::Namespaces do
         end
       end
     end
+
+    context 'with gitlab subscription' do
+      before do
+        group1.add_guest(user)
+
+        create(:gitlab_subscription, namespace: group1, max_seats_used: 1)
+      end
+
+      it 'includes max_seats_used' do
+        get api("/namespaces", user)
+
+        expect(json_response.first['max_seats_used']).to eq(1)
+      end
+    end
+
+    context 'without gitlab subscription' do
+      it 'does not include max_seats_used' do
+        get api("/namespaces", user)
+
+        json_response.each do |resp|
+          expect(resp.keys).not_to include('max_seats_used')
+        end
+      end
+    end
   end
 
   describe 'PUT /namespaces/:id' do