Fix email receiver metrics events

`Project#inspect` isn't very useful for examining usage of these features.

lib/gitlab/email/handler/create_issue_handler.rb

@@ -37,7 +37,7 @@ module Gitlab
         end
         def metrics_params
-          super.merge(project: project)
+          super.merge(project: project&.full_path)
         end
         private

lib/gitlab/email/handler/create_note_handler.rb

@@ -29,7 +29,7 @@ module Gitlab
         end
         def metrics_params
-          super.merge(project: project)
+          super.merge(project: project&.full_path)
         end
         private

lib/gitlab/email/handler/unsubscribe_handler.rb

@@ -20,7 +20,7 @@ module Gitlab
         end
         def metrics_params
-          super.merge(project: project)
+          super.merge(project: project&.full_path)
         end
         private