Expose `created_at` in group and project member response

changelogs/unreleased/expose-created-at.yml

+---
+title: Expose `created_at` in Group and Project members API response
+merge_request: 45156
+author: Rajendra Kadam
+type: added

doc/api/members.md

@@ -180,6 +180,7 @@ Example response:
   "web_url": "http://192.168.1.8:3000/root",
   "access_level": 30,
   "email": "john@example.com",
+  "created_at": "2012-10-22T14:13:35Z",
   "expires_at": null,
   "group_saml_identity": null
 }

lib/api/entities/member.rb

@@ -5,6 +5,7 @@ module API
     class Member < Grape::Entity
       expose :user, merge: true, using: UserBasic
       expose :access_level
+      expose :created_at
       expose :expires_at
     end
   end

spec/requests/api/members_spec.rb

@@ -196,6 +196,7 @@ RSpec.describe API::Members do
 
               # Member attributes
               expect(json_response['access_level']).to eq(Member::DEVELOPER)
+              expect(json_response['created_at'].to_time).to be_like_time(developer.created_at)
             end
           end
         end