Add sign_in_count to /users/:id API for admins

It makes sense to provide this information in API since it’s present in
the administration web UI.

changelogs/unreleased/add-sign_in_count-to-user-api.yml

+---
+title: Add sign_in_count to /users/:id API for admins
+merge_request: 35726
+author: Luc Didry
+type: changed

doc/api/users.md

@@ -314,7 +314,8 @@ Example Responses:
   "current_sign_in_ip": "196.165.1.102",
   "last_sign_in_ip": "172.127.2.22",
   "plan": "gold",
-  "trial": true
+  "trial": true,
+  "sign_in_count": 1337
 }
 ```
 

lib/api/entities/user_details_with_admin.rb

@@ -6,6 +6,7 @@ module API
       expose :highest_role
       expose :current_sign_in_ip
       expose :last_sign_in_ip
+      expose :sign_in_count
     end
   end
 end

spec/requests/api/users_spec.rb

@@ -64,6 +64,7 @@ RSpec.describe API::Users, :do_not_mock_admin_mode do
 
             expect(json_response).to have_key('note')
             expect(json_response['note']).to eq(user.note)
+            expect(json_response).to have_key('sign_in_count')
           end
         end
 
@@ -72,6 +73,7 @@ RSpec.describe API::Users, :do_not_mock_admin_mode do
             get api("/users/#{user.id}", user)
 
             expect(json_response).not_to have_key('note')
+            expect(json_response).not_to have_key('sign_in_count')
           end
         end
       end