Only build middleware proxy when instrumentating

The instrumentation proxy adds three stack frames per-middleware, even
when nothing is listening.

This commit, when the middleware stack is built, only adds
instrumentation when the `process_middleware.action_dispatch` event has
already been subscribed to.

The advantage to this is that we don't have any extra stack frames in
apps which don't need middleware instrumentation.

The disadvantage is that the subscriptions need to be in place when the
middleware stack is built (during app boot). I think this is likely okay
because temporary AS::Notifications subscriptions are strongly
discouraged.

actionpack/lib/action_dispatch/middleware/stack.rb

@@ -34,7 +34,11 @@ module ActionDispatch
       end
 
       def build(app)
-        InstrumentationProxy.new(klass.new(app, *args, &block), inspect)
+        klass.new(app, *args, &block)
+      end
+
+      def build_instrumented(app)
+        InstrumentationProxy.new(build(app), inspect)
       end
     end
 
@@ -119,7 +123,14 @@ module ActionDispatch
     end
 
     def build(app = nil, &block)
-      middlewares.freeze.reverse.inject(app || block) { |a, e| e.build(a) }
+      instrumenting = ActiveSupport::Notifications.notifier.listening?(InstrumentationProxy::EVENT_NAME)
+      middlewares.freeze.reverse.inject(app || block) do |a, e|
+        if instrumenting
+          e.build_instrumented(a)
+        else
+          e.build(a)
+        end
+      end
     end
 
     private

actionpack/test/controller/show_exceptions_test.rb

@@ -99,7 +99,7 @@ module ShowExceptions
   class ShowFailsafeExceptionsTest < ActionDispatch::IntegrationTest
     def test_render_failsafe_exception
       @app = ShowExceptionsOverriddenController.action(:boom)
-      middleware = @app.instance_variable_get(:@middleware)
+      middleware = @app
       @exceptions_app = middleware.instance_variable_get(:@exceptions_app)
       middleware.instance_variable_set(:@exceptions_app, nil)
       $stderr = StringIO.new

actionpack/test/dispatch/middleware_stack_test.rb

@@ -121,9 +121,6 @@ class MiddlewareStackTest < ActiveSupport::TestCase
   end
 
   test "instruments the execution of middlewares" do
-    app = @stack.build(proc { |env| [200, {}, []] })
-    env = {}
-
     events = []
 
     subscriber = proc do |*args|
@@ -131,6 +128,9 @@ class MiddlewareStackTest < ActiveSupport::TestCase
     end
 
     ActiveSupport::Notifications.subscribed(subscriber, "process_middleware.action_dispatch") do
+      app = @stack.build(proc { |env| [200, {}, []] })
+
+      env = {}
       app.call(env)
     end