Unfreeze interpolated string because it's useless.

actionpack/lib/abstract_controller/caching/fragments.rb

@@ -136,7 +136,7 @@ def expire_fragment(key, options = nil)
 
       def instrument_fragment_cache(name, key) # :nodoc:
         payload = instrument_payload(key)
-        ActiveSupport::Notifications.instrument("#{name}.#{instrument_name}".freeze, payload) { yield }
+        ActiveSupport::Notifications.instrument("#{name}.#{instrument_name}", payload) { yield }
       end
     end
   end

actionview/lib/action_view/template.rb

@@ -345,7 +345,7 @@ def identifier_method_name
       end
 
       def instrument(action, &block) # :doc:
-        ActiveSupport::Notifications.instrument("#{action}.action_view".freeze, instrument_payload, &block)
+        ActiveSupport::Notifications.instrument("#{action}.action_view", instrument_payload, &block)
       end
 
       def instrument_render_template(&block)