Add new Rack instrumentation middleware

Let's no longer the GenericInstrumentation middleware with its
default action name set to `unknown`.

This new InstrumentationMiddleware will not report any action name, only
report an instrumentation event.

The action name can be set by the app using the `Appsignal.set_action`
helper from now on in this middleware.

This will require us to update the Rack docs page to replace
GenericInstrumentation with InstrumentationMiddleware.

.changesets/add-new-recommended-rack-instrumentation-middleware.md

@@ -0,0 +1,18 @@
+---
+bump: minor
+type: add
+---
+
+Add our new recommended Rack instrumentation middleware. If an app is using the `Appsignal::Rack::GenericInstrumentation` middleware, please update it to use `Appsignal::Rack::InstrumentationMiddleware` instead.
+
+This new middleware will not report all requests under the "unknown" action if no action name is set. To set an action name, call the `Appsignal.set_action` helper from the app.
+
+```ruby
+# config.ru
+
+# Setup AppSignal
+
+use Appsignal::Rack::InstrumentationMiddleware
+
+# Run app
+```

lib/appsignal.rb

@@ -325,6 +325,7 @@ def const_missing(name)
 require "appsignal/marker"
 require "appsignal/garbage_collection"
 require "appsignal/rack/abstract_middleware"
+require "appsignal/rack/instrumentation_middleware"
 require "appsignal/rack/generic_instrumentation"
 require "appsignal/rack/event_handler"
 require "appsignal/integrations/railtie" if defined?(::Rails)

lib/appsignal/rack/abstract_middleware.rb

@@ -72,8 +72,8 @@ def call(env)
       # don't report any exceptions here, the top instrumentation middleware
       # will be the one reporting the exception.
       #
-      # Either another {GenericInstrumentation} or {EventHandler} is higher in
-      # the stack and will report the exception and complete the transaction.
+      # Either another {AbstractMiddleware} or {EventHandler} is higher in the
+      # stack and will report the exception and complete the transaction.
       #
       # @see {#instrument_app_call_with_exception_handling}
       def instrument_app_call(env)

lib/appsignal/rack/instrumentation_middleware.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+module Appsignal
+  module Rack
+    # @api public
+    class InstrumentationMiddleware < AbstractMiddleware
+      def initialize(app, options = {})
+        options[:instrument_span_name] ||= "process_request_middleware.rack"
+        super
+      end
+    end
+  end
+end

spec/lib/appsignal/rack/instrumentation_middleware_spec.rb

@@ -0,0 +1,38 @@
+describe Appsignal::Rack::InstrumentationMiddleware do
+  let(:app) { DummyApp.new }
+  let(:env) { Rack::MockRequest.env_for("/some/path") }
+  let(:middleware) { described_class.new(app, {}) }
+
+  before { start_agent }
+  around { |example| keep_transactions { example.run } }
+
+  def make_request(env)
+    middleware.call(env)
+  end
+
+  context "without an exception" do
+    it "reports a process_request_middleware.rack event" do
+      make_request(env)
+
+      expect(last_transaction).to include_event("name" => "process_request_middleware.rack")
+    end
+  end
+
+  context "with custom action name" do
+    let(:app) { DummyApp.new { |_env| Appsignal.set_action("MyAction") } }
+
+    it "reports the custom action name" do
+      make_request(env)
+
+      expect(last_transaction).to have_action("MyAction")
+    end
+  end
+
+  context "without action name metadata" do
+    it "reports no action name" do
+      make_request(env)
+
+      expect(last_transaction).to_not have_action
+    end
+  end
+end