Fetch payload body from params[:notification]

Fetch payload body from `params[:notification]` instead of slicing out
service params such as controller, and action names.

If the "Content-Type" header of your request is set
to "application/json", Rails will automatically load your parameters
into the params hash, which you can access as you would normally.

https://guides.rubyonrails.org/v5.2/
action_controller_overview.html#json-parameters

ee/app/controllers/projects/alerting/notifications_controller.rb

@@ -19,8 +19,6 @@ module Projects
 
       private
 
-      PARAMS_TO_EXCLUDE = %w(controller action namespace_id project_id).freeze
-
       def project_without_auth
         @project ||= Project
           .find_by_full_path("#{params[:namespace_id]}/#{params[:project_id]}")
@@ -36,7 +34,7 @@ module Projects
 
       def notify_service
         Projects::Alerting::NotifyService
-          .new(project, current_user, permitted_params)
+          .new(project, current_user, notification_payload)
       end
 
       def response_status(result)
@@ -45,8 +43,8 @@ module Projects
         result.http_status
       end
 
-      def permitted_params
-        params.except(*PARAMS_TO_EXCLUDE).permit!
+      def notification_payload
+        params.permit![:notification]
       end
     end
   end

ee/spec/controllers/projects/alerting/notifications_controller_spec.rb

@@ -14,8 +14,8 @@ describe Projects::Alerting::NotificationsController do
       allow(Projects::Alerting::NotifyService).to receive(:new).and_return(notify_service)
     end
 
-    def make_request(opts = {})
-      post :create, params: project_params(opts), session: { as: :json }
+    def make_request(body = {})
+      post :create, params: project_params, body: body.to_json, as: :json
     end
 
     context 'when feature flag is on' do