Test the adding of custom fields via LogStasher

The way LogStasher hooks into Rails makes it difficult to test that
configured custom fields actually get added.

It was tricky to find an approach to worked, wasn't too awkward and
wasn't too tightly coupled to LogStasher.

_Notes on some of the esoteric details of these new tests_

`append_info_to_payload`:

This is a Rails instrumentation hook (intended for use by third
parties). It's run during request handling and LogStasher uses it to
"append" its default fields and any custom fields defined by our app.
The visible effect in the context of this method and its caller is that
those LogStasher fields are set on the `payload` hash that was supplied
to the method.

So, invoking this method is enough to simulate a controller action
running for LogStasher's purposes.

And, if the payload hash contains our custom field key (and value, when
there is one), we've defined it successfully.

`logstasher_add_custom_fields_to_payload`:

If a Rails app has defined custom LogStasher fields (with
`LogStasher.add_custom_fields`), LogStasher registers a listener with
Rails that fires whenever a controller is loaded and that listener adds
this method to the loaded controller.

Therefore, if a newly-loaded controller has this method, one or more
LogStasher custom fields have been defined successfully.

spec/lib/govuk_json_logging_spec.rb

@@ -3,6 +3,11 @@
 require "govuk_app_config/govuk_json_logging"
 require "rack/test"
 
+class TestController < ActionController::Base
+  include LogStasher::ActionController::Instrumentation
+  def index; end
+end
+
 RSpec.describe GovukJsonLogging do
   let(:govuk_headers_class) do
     Class.new do
@@ -78,6 +83,66 @@ def self.headers
       expect(fake_stdout.read).to match(/test default log entry/)
     end
 
+    context "given a block" do
+      it "evals the block" do
+        done = false
+        expect {
+          GovukJsonLogging.configure do
+            done = true
+          end
+        }.to change { done }.to(true)
+      end
+
+      context "and the block configures custom fields" do
+        describe "any subsequently-created ActionController" do
+          let(:headers) { { "REMOTE_ADDR" => "10.10.10.10" } }
+          let(:mock_request) { ActionDispatch::TestRequest.new(Rack::MockRequest.env_for("http://example.com:8080/", headers)) }
+          let(:mock_response) { ActionDispatch::TestResponse.new }
+
+          before do
+            GovukJsonLogging.configure do
+              add_custom_fields do |fields|
+                fields[:govuk_custom_field] = request.headers["GOVUK-Custom-Header"]
+              end
+            end
+
+            @controller = TestController.new
+            allow(@controller).to receive(:request).and_return(mock_request)
+            allow(@controller).to receive(:response).and_return(mock_response)
+          end
+
+          it "has a logstasher_add_custom_fields_to_payload method" do
+            expect(@controller.methods).to include(:logstasher_add_custom_fields_to_payload)
+          end
+
+          describe "calling the logstasher_add_custom_fields_to_payload" do
+            let(:payload) { {} }
+
+            it "executes the block" do
+              expect(@controller).to receive(:logstasher_add_custom_fields_to_payload)
+              @controller.send(:append_info_to_payload, payload)
+            end
+
+            it "adds the custom fields to the payload" do
+              @controller.send(:append_info_to_payload, payload)
+              expect(payload.keys).to include(:govuk_custom_field)
+            end
+
+            context "when the custom field has a value" do
+              before do
+                mock_request.headers["GOVUK-Custom-header"] = "My header value"
+              end
+
+              it "sets the custom field value in the payload" do
+                @controller.send(:append_info_to_payload, payload)
+                expect(payload[:govuk_custom_field]).to eq("My header value")
+              end
+            end
+          end
+        end
+      end
+    end
+
     describe "when making requests to the application" do
       include Rack::Test::Methods