Add the action instance to the request environment

I created a [new topic] on the Hanami Discourse about improving the
AppSignal APM instrumentation.

To group requests, we would like to know what action it took place in.
This information is currently not available in the request environment.

For us, the easiest would be to access the action instance. That gives
us the class information and a way to access the `params_class` to fetch
all the parameters of the request: query params and the body payload.

This change adds the action instance on a new request environment key
`hanami.action_instance`.

[new topic]: https://discourse.hanamirb.org/t/questions-for-improving-the-appsignal-apm-integration-with-hanami-2/989/3

I had to update one test to not fail on the action instance being
returned, which is not important for that spec I think.

Closes hanami#445

lib/hanami/action.rb

@@ -301,6 +301,7 @@ def initialize(config: self.class.config)
     # @since 0.1.0
     # @api private
     def call(env)
+      env[ACTION_INSTANCE] = self
       request  = nil
       response = nil
 

lib/hanami/action/constants.rb

@@ -245,5 +245,9 @@ class Action
     # @since 2.0.0
     # @api private
     DEFAULT_CHARSET = "utf-8"
+
+    # @since 2.2.0
+    # @api private
+    ACTION_INSTANCE = "hanami.action_instance"
   end
 end

spec/support/fixtures.rb

@@ -585,7 +585,9 @@ def handle(_req, _res)
 
 class ParamsAction < Hanami::Action
   def handle(req, res)
-    res.body = req.params.to_h.inspect
+    params = req.params.to_h
+    params.delete(Hanami::Action::ACTION_INSTANCE.to_sym)
+    res.body = params.inspect
   end
 end
 

spec/unit/hanami/action_spec.rb

@@ -21,6 +21,14 @@
       expect(response.body).to    eq(["Hi from TestAction!"])
     end
 
+    it "sets the action instance on the request environment object" do
+      action = CallAction.new
+      env = {}
+      action.call(env)
+
+      expect(env[Hanami::Action::ACTION_INSTANCE]).to eq(action)
+    end
+
     context "when an exception isn't handled" do
       it "should raise an actual exception" do
         expect { UncheckedErrorCallAction.new.call({}) }.to raise_error(RuntimeError)