feat(rack)! Use Rack Events By Default

I have been running the `Rack::Events` based instrumentation in our production workloads since 2023-10-23.

There is no noticeable difference in performance, however applications using `Rack::BodyProxy`
to stream responses will notice a slight increase in latency since it will include timings
writing response output to the socket.

instrumentation/rack/example/trace_demonstration.rb

@@ -18,7 +18,7 @@
 builder = Rack::Builder.app do
   # integration should be automatic in web frameworks (like rails),
   # but for a plain Rack application, enable it in your config.ru, e.g.,
-  use OpenTelemetry::Instrumentation::Rack::Middlewares::TracerMiddleware
+  use ::Rack::Events, [OpenTelemetry::Instrumentation::Rack::Middlewares::EventHandler.new]
 
   app = ->(_env) { [200, { 'Content-Type' => 'text/plain' }, ['All responses are OK']] }
   run app

instrumentation/rack/example/trace_demonstration2.rb

@@ -24,7 +24,7 @@
 end
 
 # integrate instrumentation explicitly:
-builder.use OpenTelemetry::Instrumentation::Rack::Middlewares::TracerMiddleware
+builder.use ::Rack::Events, [OpenTelemetry::Instrumentation::Rack::Middlewares::EventHandler.new]
 
 # demonstrate tracing (span output to console):
 puts Rack::MockRequest.new(builder).get('/')

instrumentation/rack/example/trace_demonstration3.rb

@@ -19,7 +19,7 @@
 builder = Rack::Builder.app do
   # integration should be automatic in web frameworks (like rails),
   # but for a plain Rack application, enable it in your config.ru, e.g.,
-  use OpenTelemetry::Instrumentation::Rack::Middlewares::TracerMiddleware
+  use ::Rack::Events, [OpenTelemetry::Instrumentation::Rack::Middlewares::EventHandler.new]
 
   app = ->(_env) { [200, { 'Content-Type' => 'text/plain' }, ['All responses are OK']] }
   run app

instrumentation/rack/lib/opentelemetry/instrumentation/rack/instrumentation.rb

@@ -29,7 +29,7 @@ class Instrumentation < OpenTelemetry::Instrumentation::Base
         option :untraced_requests,        default: nil,   validate: :callable
         option :response_propagators,     default: [],    validate: :array
         # This option is only valid for applicaitons using Rack 2.0 or greater
-        option :use_rack_events,          default: false, validate: :boolean
+        option :use_rack_events,          default: true, validate: :boolean
 
         # Temporary Helper for Sinatra and ActionPack middleware to use during installation
         #

instrumentation/rack/test/opentelemetry/instrumentation/rack/instrumentation_test.rb

@@ -32,7 +32,7 @@
       _(instrumentation.config[:url_quantization]).must_be_nil
       _(instrumentation.config[:untraced_requests]).must_be_nil
       _(instrumentation.config[:response_propagators]).must_be_empty
-      _(instrumentation.config[:use_rack_events]).must_equal false
+      _(instrumentation.config[:use_rack_events]).must_equal true
     end
   end