Freeze fragment cache related instrument name.

ActionMailer::Base#instrument_name and
ActionController::Base#instrument_name will be frequently called once
caching is enabled. So it's better to freeze them instead of create new
string on every call.

Also, the instrument name in #instrument_fragment_cache will usually
be "write_fragment.action_controller" or
"read_fragment.action_controller". So freezing them might also gain some
performance improvement. We have done something like this in other places:
https://github.com/rails/rails/blob/master/actionview/lib/action_view/template.rb#L348

1 files changed, 1 insertions, 1 deletions

diff --git a/actionmailer/lib/action_mailer/base.rb b/actionmailer/lib/action_mailer/base.rb
index 444be944df..9b5d39faea 100644
--- a/actionmailer/lib/action_mailer/base.rb
+++ b/actionmailer/lib/action_mailer/base.rb
@@ -972,7 +972,7 @@ module ActionMailer
       end
 
       def instrument_name
-        "action_mailer"
+        "action_mailer".freeze
       end
 
       ActiveSupport.run_load_hooks(:action_mailer, self)