Add document_type and schema_name to prom labels

This results in metrics like:

    http_requests_total{action="show",controller="content_items",document_type="answer",schema_name="answer",status="200"} 1

This will allow us to investigate the performance of different content
types, for example how long does it take to load a manual vs. an answer
or a case_study. Currently we can't distinguish these in the metrics for
government-frontend, because it uses the same controller action for all
content types.

If there are some content types which take a much longer time to load,
maybe there are performance issues we should address.

This follows on from alphagov/govuk_app_config#424
which adds the functionality to support setting prometheus labels in this way.

app/controllers/content_items_controller.rb

@@ -25,6 +25,7 @@ def show
     load_content_item
 
     set_expiry
+    set_prometheus_labels
 
     if is_service_manual?
       show_service_manual_page
@@ -234,6 +235,14 @@ def set_expiry
     )
   end
 
+  def set_prometheus_labels
+    prometheus_labels = request.env.fetch("govuk.prometheus_labels", {})
+    request.env["govuk.prometheus_labels"] = prometheus_labels.merge(
+      document_type: @content_item.document_type,
+      schema_name: @content_item.schema_name,
+    )
+  end
+
   def service_url(original_url)
     ga_param = params[:_ga]
     return original_url if ga_param.nil?

test/controllers/content_items_controller_test.rb

@@ -158,6 +158,14 @@ class ContentItemsControllerTest < ActionController::TestCase
     assert_equal content_item["title"], assigns[:content_item].title
   end
 
+  test "sets prometheus labels on the rack env" do
+    content_item = content_store_has_schema_example("case_study", "case_study")
+
+    get :show, params: { path: path_for(content_item) }
+    assert_response :success
+    assert_equal @request.env["govuk.prometheus_labels"], { document_type: "case_study", schema_name: "case_study" }
+  end
+
   test "gets item from content store and keeps existing ordered_related_items when links already exist" do
     content_item = content_store_has_schema_example("guide", "guide")