Use ErrorTracking::ClientKey when deleting a key

This change alters the response when deleting a client key within the
error tracking feature. To avoid leaking unnecessary fields, we use the
Entities::ErrorTracking::ClientKey to serialize the deleted object.

lib/api/error_tracking/client_keys.rb

@@ -43,6 +43,8 @@ class ErrorTracking::ClientKeys < ::API::Base
         delete '/client_keys/:key_id' do
           key = user_project.error_tracking_client_keys.find(params[:key_id])
           key.destroy!
+
+          present key, with: Entities::ErrorTracking::ClientKey
         end
       end
     end

spec/requests/api/error_tracking/client_keys_spec.rb

@@ -81,6 +81,10 @@
       it 'returns a correct status' do
         expect(response).to have_gitlab_http_status(:ok)
       end
+
+      it 'returns specific fields using the entity' do
+        expect(json_response.keys).to match_array(%w[id active public_key sentry_dsn])
+      end
     end
   end
 end