Disable project metric label

Having project label causes significant cardinality in service desk
metrics, second problem is that because these service desk events are
relatively rare, there are not enough events for some pods and data in
rate graphs are misrepresented.

lib/gitlab/email/handler/service_desk_handler.rb

@@ -42,10 +42,6 @@ module Gitlab
           end
         end
 
-        def metrics_params
-          super.merge(project: project&.full_path)
-        end
-
         def metrics_event
           :receive_email_service_desk
         end

spec/lib/gitlab/email/handler/service_desk_handler_spec.rb

@@ -54,7 +54,7 @@ RSpec.describe Gitlab::Email::Handler::ServiceDeskHandler do
       it 'adds metric events for incoming and reply emails' do
         metric_transaction = double('Gitlab::Metrics::WebTransaction', increment: true, observe: true)
         allow(::Gitlab::Metrics::BackgroundTransaction).to receive(:current).and_return(metric_transaction)
-        expect(metric_transaction).to receive(:add_event).with(:receive_email_service_desk, anything)
+        expect(metric_transaction).to receive(:add_event).with(:receive_email_service_desk, { handler: 'Gitlab::Email::Handler::ServiceDeskHandler' })
         expect(metric_transaction).to receive(:add_event).with(:service_desk_thank_you_email)
 
         receiver.execute