Report request response status for Rails apps

We can now report the response status as a `response_status` tag and
metric. Previously, due to the position of the Rails middleware, this
was unreliable and wouldn't always report the real status code if other
middleware in the stack that are run after ours returned another status.

This is part of issue #183, but only implements it now for Rails apps
and apps using the `Appsignal::Rack::EventHandler` manually. We'll need
to update other instrumentations for Rack, Sinatra, Padrino, Grape, etc.
to use this new EventHandler, so they can also benefit from this new
response status tag and metric.

.changesets/report-response-status-for-rails-requests.md

@@ -0,0 +1,8 @@
+---
+bump: patch
+type: add
+---
+
+Report the response status for Rails requests as the `response_status` tag on samples, e.g. 200, 301, 500. This tag is visible on the sample detail page.
+
+The response status is also reported as the `response_status` metric.

lib/appsignal/rack/event_handler.rb

@@ -53,8 +53,16 @@ def on_finish(request, _response)
 
         transaction.finish_event("process_request.rack", "", "")
         transaction.set_action_if_nil("#{request.request_method} #{request.path}")
+        transaction.set_tags(:response_status => response.status)
         transaction.set_http_or_background_queue_start
 
+        Appsignal.increment_counter(
+          :response_status,
+          1,
+          :status => response.status,
+          :namespace => format_namespace(transaction.namespace)
+        )
+
         # Make sure the current transaction is always closed when the request
         # is finished
         Appsignal::Transaction.complete_current!

spec/lib/appsignal/rack/event_handler_spec.rb

@@ -160,6 +160,25 @@ def on_finish
       )
     end
 
+    it "sets the response status as a tag" do
+      on_start
+      on_finish
+
+      expect(last_transaction.to_h).to include(
+        "sample_data" => hash_including(
+          "tags" => { "response_status" => 200 }
+        )
+      )
+    end
+
+    it "increments the response status counter for response status" do
+      expect(Appsignal).to receive(:increment_counter)
+        .with(:response_status, 1, :status => 200, :namespace => :web)
+
+      on_start
+      on_finish
+    end
+
     it "logs an error in case of an error" do
       expect(Appsignal::Transaction)
         .to receive(:complete_current!).and_raise(ExampleStandardError, "oh no")