Merge branch 'rc/add_back_sidekiq_exporter' into 'master'

Add back sidekiq metrics exporter

See merge request gitlab-org/gitlab-ce!29647

config/initializers/7_prometheus_metrics.rb

@@ -19,6 +19,12 @@ Gitlab::Application.configure do |config|
   config.middleware.insert(1, Gitlab::Metrics::RequestsRackMiddleware)
 end
 
+Sidekiq.configure_server do |config|
+  config.on(:startup) do
+    Gitlab::Metrics::SidekiqMetricsExporter.instance.start
+  end
+end
+
 if !Rails.env.test? && Gitlab::Metrics.prometheus_metrics_enabled?
   Gitlab::Cluster::LifecycleEvents.on_worker_start do
     defined?(::Prometheus::Client.reinitialize_on_pid_change) && Prometheus::Client.reinitialize_on_pid_change