Support nested webmachine apps

Apply the same logic as we have in the AbstractMiddleware for webmachine
apps. When a parent transaction is active, use that and don't create a
new one. More importantly, don't close the active transaction.

The webmachine instrumentation can't use the AbstractMiddleware, or any
middleware, as this discouraged by the webmachine project. In practice I
don't see this happening, but if someone happens to add an AppSignal
EventHandler to the middleware stack in `config.ru`, we now support it.

Part of #329

.changesets/support-nested-webmachine-apps.md

@@ -0,0 +1,6 @@
+---
+bump: patch
+type: add
+---
+
+Support nested webmachine apps. If webmachine apps are nested in other AppSignal instrumentation it will now report the webmachine instrumentation as part of the parent transaction, reporting more runtime of the request.

lib/appsignal/integrations/webmachine.rb

@@ -5,20 +5,26 @@ module Integrations
     # @api private
     module WebmachineIntegration
       def run
-        transaction = Appsignal::Transaction.create(
-          SecureRandom.uuid,
-          Appsignal::Transaction::HTTP_REQUEST,
-          request,
-          :params_method => :query
-        )
-
-        transaction.set_action_if_nil("#{resource.class.name}##{request.method}")
+        has_parent_transaction = Appsignal::Transaction.current?
+        transaction =
+          if has_parent_transaction
+            Appsignal::Transaction.current
+          else
+            Appsignal::Transaction.create(
+              SecureRandom.uuid,
+              Appsignal::Transaction::HTTP_REQUEST,
+              request
+            )
+          end
 
         Appsignal.instrument("process_action.webmachine") do
           super
         end
+      ensure
+        transaction.set_action_if_nil("#{resource.class.name}##{request.method}")
+        transaction.set_params_if_nil(request.query)
 
-        Appsignal::Transaction.complete_current!
+        Appsignal::Transaction.complete_current! unless has_parent_transaction
       end
 
       private

spec/lib/appsignal/integrations/webmachine_spec.rb

@@ -86,6 +86,26 @@ def to_html
         expect(last_transaction).to be_completed
         expect(current_transaction?).to be_falsy
       end
+
+      context "with parent transaction" do
+        let(:transaction) { http_request_transaction }
+        before { set_current_transaction(transaction) }
+
+        it "sets the action" do
+          fsm.run
+          expect(last_transaction).to have_action("MyResource#GET")
+        end
+
+        it "sets the params" do
+          fsm.run
+          last_transaction._sample
+          expect(last_transaction).to include_params("param1" => "value1", "param2" => "value2")
+        end
+
+        it "does not close the transaction" do
+          expect(last_transaction).to_not be_completed
+        end
+      end
     end
 
     describe "#handle_exceptions" do