Re-position CaptureExceptions middleware

When `CaptureExceptions` is the first middleware, the benefits are:

1. It captures as many exceptions as possible.
2. Transactions can record almost the full length of the request's
   events.

But there are also some downsides:

1. Exceptions of the outter middlewares may mostly be noise to users, as
   they usually are the Rails default.
2. It makes skipping some requests' transactions very hard. For example:
  #1404

By re-positioning the middleware after `ActionDispatch::Executor`, we
can automatically avoid the downsides while still keeping most of the
benefits. It also allows us to remove the patch on `FileHandler`.

sentry-rails/lib/sentry/rails/railtie.rb

@@ -6,8 +6,8 @@ module Sentry
   class Railtie < ::Rails::Railtie
     # middlewares can't be injected after initialize
     initializer "sentry.use_rack_middleware" do |app|
-      # need to be placed at first to capture as many errors as possible
-      app.config.middleware.insert 0, Sentry::Rails::CaptureExceptions
+      # placed after all the file-sending middlewares so we can avoid unnecessary transactions
+      app.config.middleware.insert_after ActionDispatch::Executor, Sentry::Rails::CaptureExceptions
       # need to be placed at last to smuggle app exceptions via env
       app.config.middleware.use(Sentry::Rails::RescuedExceptionInterceptor)
     end
@@ -21,7 +21,6 @@ class Railtie < ::Rails::Railtie
       extend_active_job if defined?(ActiveJob)
       patch_background_worker if defined?(ActiveRecord)
       override_streaming_reporter if defined?(ActionView)
-      override_file_handler if defined?(ActionDispatch) && app.config.public_file_server.enabled
       setup_backtrace_cleanup_callback
       inject_breadcrumbs_logger
       activate_tracing
@@ -79,14 +78,6 @@ def override_streaming_reporter
       end
     end
 
-    def override_file_handler
-      require "sentry/rails/overrides/file_handler"
-
-      ActiveSupport.on_load :action_controller do
-        ActionDispatch::FileHandler.send(:prepend, Sentry::Rails::Overrides::FileHandler)
-      end
-    end
-
     def activate_tracing
       if Sentry.configuration.tracing_enabled?
         subscribers = Sentry.configuration.rails.tracing_subscribers

sentry-rails/spec/sentry/rails/tracing_spec.rb

@@ -112,10 +112,16 @@
   end
 
   context "with config.public_file_server.enabled = true" do
+    let(:string_io) { StringIO.new }
+    let(:logger) do
+      ::Logger.new(string_io)
+    end
+
     before do
       make_basic_app do |config, app|
         app.config.public_file_server.enabled = true
         config.traces_sample_rate = 1.0
+        config.logger = logger
       end
     end
 
@@ -124,6 +130,7 @@
 
       expect(response).to have_http_status(:ok)
       expect(transport.events).to be_empty
+      expect(string_io.string).not_to match(/\[Tracing\] Starting <rails\.request>/)
     end
 
     it "doesn't get messed up by previous exception" do

sentry-rails/spec/sentry/rails_spec.rb

@@ -24,7 +24,7 @@
 
     it "inserts middleware to a correct position" do
       app = Rails.application
-      expect(app.middleware.find_index(Sentry::Rails::CaptureExceptions)).to eq(0)
+      expect(app.middleware.find_index(Sentry::Rails::CaptureExceptions)).to eq(4)
       expect(app.middleware.find_index(Sentry::Rails::RescuedExceptionInterceptor)).to eq(app.middleware.count - 1)
     end