Add instrumentation support

lib/premailer/rails/railtie.rb

@@ -2,7 +2,9 @@ class Premailer
   module Rails
     class Railtie < ::Rails::Railtie
       ActiveSupport.on_load(:action_mailer) do
-        ::Premailer::Rails.register_interceptors
+        ActiveSupport::Notifications.instrument('register_interceptors.premailer_rails') do
+          ::Premailer::Rails.register_interceptors
+        end
       end
     end
   end

spec/railtie_spec.rb

@@ -0,0 +1,15 @@
+require 'spec_helper'
+
+describe Premailer::Rails::Railtie do
+  it 'supports instrumentation' do
+    subscriber = double().as_null_object
+
+    ActiveSupport::Notifications.subscribe('register_interceptors.premailer_rails') do
+      subscriber.call
+    end
+
+    ActiveSupport.run_load_hooks(:action_mailer, ActionMailer::Base)
+
+    expect(subscriber).to have_received(:call).at_least(:once)
+  end
+end