Instrument each load config initializer

In order to simplify profiling loading of initializers,
added instument for tracking load config initializer event from
`config/initializers`

guides/source/active_support_instrumentation.md

@@ -396,6 +396,15 @@ INFO. Cache stores my add their own keys
 }
 ```
 
+Railties
+--------
+
+### load_config_initializer.railties
+
+| Key            | Value                                                 |
+| -------------- | ----------------------------------------------------- |
+| `:initializer` | Path to loaded initializer from `config/initializers` |
+
 Rails
 -----
 

railties/CHANGELOG.md

@@ -1,3 +1,33 @@
+*   Instrument an `load_config_initializer.railties` event on each load of configuration initializer
+    from `config/initializers`. Subscribers should be attached before `load_config_initializers`
+    initializer completed.
+
+    Registering subscriber examples:
+
+        # config/application.rb
+        module RailsApp
+          class Application < Rails::Application
+            ActiveSupport::Notifications.subscribe('load_config_initializer.railties') do |*args|
+              event = ActiveSupport::Notifications::Event.new(*args)
+              puts "Loaded initializer #{event.payload[:initializer]} (#{event.duration}ms)"
+            end
+          end
+        end
+
+        # my_engine/lib/my_engine/engine.rb
+        module MyEngine
+          class Engine < ::Rails::Engine
+            config.before_initialize do
+              ActiveSupport::Notifications.subscribe('load_config_initializer.railties') do |*args|
+                event = ActiveSupport::Notifications::Event.new(*args)
+                puts "Loaded initializer #{event.payload[:initializer]} (#{event.duration}ms)"
+              end
+            end
+          end
+        end
+
+    *Paul Nikitochkin*
+
 *   Support for Pathnames in eager load paths.
 
     *Mike Pack*

railties/lib/rails/engine.rb

@@ -611,7 +611,7 @@ module Rails
 
     initializer :load_config_initializers do
       config.paths["config/initializers"].existent.sort.each do |initializer|
-        load(initializer)
+        load_config_initializer(initializer)
       end
     end
 
@@ -645,6 +645,12 @@ module Rails
 
     protected
 
+    def load_config_initializer(initializer)
+      ActiveSupport::Notifications.instrument('load_config_initializer.railties', initializer: initializer) do
+        load(initializer)
+      end
+    end
+
     def run_tasks_blocks(*) #:nodoc:
       super
       paths["lib/tasks"].existent.sort.each { |ext| load(ext) }

railties/test/application/initializers/notifications_test.rb

@@ -39,5 +39,18 @@ module ApplicationTests
       assert_equal 1, logger.logged(:debug).size
       assert_match(/SHOW tables/, logger.logged(:debug).last)
     end
+
+    test 'rails load_config_initializer event is instrumented' do
+      app_file 'config/initializers/foo.rb', ''
+
+      events = []
+      callback = -> (*_) { events << _ }
+      ActiveSupport::Notifications.subscribed(callback, 'load_config_initializer.railties') do
+        app
+      end
+
+      assert_equal %w[load_config_initializer.railties], events.map(&:first)
+      assert_includes events.first.last[:initializer], 'config/initializers/foo.rb'
+    end
   end
 end