Add Sidekiq Prometheus instrumentation

If Sidekiq is present, add the Sidekiq Prometheus instrumentation.

Ref:
1. [trello card](https://trello.com/c/K8aUtwz4/974-graphs-for-sidekiq-queues-in-grafana)

lib/govuk_app_config.rb

@@ -6,9 +6,9 @@
 # This require is deprecated and should be removed on next major version bump
 # and should be required by applications directly.
 require "govuk_app_config/govuk_unicorn"
+require "govuk_app_config/govuk_prometheus_exporter"
 
 if defined?(Rails)
-  require "govuk_app_config/govuk_prometheus_exporter"
   require "govuk_app_config/govuk_logging"
   require "govuk_app_config/govuk_content_security_policy"
   require "govuk_app_config/railtie"

lib/govuk_app_config/govuk_prometheus_exporter.rb

@@ -1,14 +1,34 @@
 module GovukPrometheusExporter
   def self.configure
-    unless Rails.env == "test" || (ENV["GOVUK_PROMETHEUS_EXPORTER"]) != "true"
+    if (ENV["GOVUK_PROMETHEUS_EXPORTER"]) == "true"
+      return if defined?(Rails) ? Rails.env == "test" : false
+
       require "prometheus_exporter"
       require "prometheus_exporter/server"
       require "prometheus_exporter/middleware"
 
+      if defined?(Sidekiq)
+        Sidekiq.configure_server do |config|
+          require "prometheus_exporter/instrumentation"
+          config.server_middleware do |chain|
+            chain.add PrometheusExporter::Instrumentation::Sidekiq
+          end
+          config.death_handlers << PrometheusExporter::Instrumentation::Sidekiq.death_handler
+          config.on :startup do
+            PrometheusExporter::Instrumentation::Process.start type: "sidekiq"
+            PrometheusExporter::Instrumentation::SidekiqProcess.start
+            PrometheusExporter::Instrumentation::SidekiqQueue.start
+            PrometheusExporter::Instrumentation::SidekiqStats.start
+          end
+        end
+      end
+
       server = PrometheusExporter::Server::WebServer.new bind: "0.0.0.0", port: 9394
       server.start
 
-      Rails.application.middleware.unshift PrometheusExporter::Middleware
+      if defined?(Rails)
+        Rails.application.middleware.unshift PrometheusExporter::Middleware
+      end
     end
   end
 end