Add the vertex response time to the response header

This is a bit of a workaround so that we can see vertex response
time for individual queries in the kibana logs.

However it looks like there is still an issue with the logstasher
initialisation - running the app in docker I can see errors that
were supposed to have been fixed by

https://github.com/alphagov/govuk_app_config/pull/561/changes
https://github.com/alphagov/govuk_app_config/pull/564/changes

Author: hannako

app/controllers/searches_controller.rb

@@ -1,10 +1,12 @@
 class SearchesController < ApplicationController
   before_action :validate_query_params, only: :show
+  after_action :add_response_time_header
 
   def show
-    ActiveSupport::Notifications.instrument("vertex_search_request_duration") do
-      render json: DiscoveryEngine::Query::Search.new(query_params, user_agent: request.user_agent).result_set
+    with_duration_logging do
+      @search_results = DiscoveryEngine::Query::Search.new(query_params, user_agent: request.user_agent).result_set
     end
+    render json: @search_results
   end
 
 private
@@ -16,4 +18,14 @@ def query_params
   def validate_query_params
     raise ActionController::BadRequest, "Invalid query parameter" unless params.fetch(:q, "").is_a?(String)
   end
+
+  def with_duration_logging(&block)
+    @duration = Benchmark.realtime(&block)
+  end
+
+  def add_response_time_header
+    return unless @duration
+
+    response.headers["Vertex-Response-Time"] = @duration.round(4).to_s
+  end
 end

config/initializers/logstasher.rb

@@ -1,11 +1,7 @@
 if ENV["GOVUK_RAILS_JSON_LOGGING"].present?
-  ActiveSupport::Notifications.subscribe("vertex_search_request_duration") do |event|
-    duration_in_seconds = event.duration / 1000.0
-
-    GovukJsonLogging.configure do
-      add_custom_fields do |fields|
-        fields[:vertex_search_duration] = duration_in_seconds
-      end
+  GovukJsonLogging.configure do
+    add_custom_fields do |fields|
+      fields[:vertex_search_duration] = response.headers["Vertex-Response-Time"]
     end
   end
 end

spec/requests/search_request_spec.rb

@@ -23,6 +23,11 @@
       })
     end
 
+    it "adds a vertex response time header to the response" do
+      get "/search.json"
+      expect(response.headers["vertex-response-time"]).not_to be_nil
+    end
+
     it "passes any query parameters and user agent to the search service in the expected format" do
       get "/search.json?q=garden+centres&start=11&count=22&filter_public_timestamp=from:2019-01-01",
           headers: { "User-Agent" => "gds-api-adapters/99.2.0 (finder-frontend)" }