Add support for using RequestStore within Sidekiq tasks via SIDEKIQ_REQUEST_STORE env variable

This significantly reduces the DB churn in the PostReceive task when it
performs reference extraction.

See #18663

CHANGELOG

@@ -4,6 +4,7 @@ v 8.11.0 (unreleased)
   - Remove magic comments (`# encoding: UTF-8`) from Ruby files !5456 (winniehell)
   - Fix CI status icon link underline (ClemMakesApps)
   - Fix of 'Commits being passed to custom hooks are already reachable when using the UI'
+  - Add support for using RequestStore within Sidekiq tasks via SIDEKIQ_REQUEST_STORE env variable
   - Limit git rev-list output count to one in forced push check
   - Add green outline to New Branch button !5447 (winniehell)
   - Retrieve rendered HTML from cache in one request

config/initializers/sidekiq.rb

@@ -7,6 +7,7 @@ Sidekiq.configure_server do |config|
   config.server_middleware do |chain|
     chain.add Gitlab::SidekiqMiddleware::ArgumentsLogger if ENV['SIDEKIQ_LOG_ARGUMENTS']
     chain.add Gitlab::SidekiqMiddleware::MemoryKiller if ENV['SIDEKIQ_MEMORY_KILLER_MAX_RSS']
+    chain.add Gitlab::SidekiqMiddleware::RequestStoreMiddleware if ENV['SIDEKIQ_REQUEST_STORE']
   end
 
   # Sidekiq-cron: load recurring jobs from gitlab.yml

lib/gitlab/sidekiq_middleware/request_store_middleware.rb

+module Gitlab
+  module SidekiqMiddleware
+    class RequestStoreMiddleware
+      def call(worker, job, queue)
+        RequestStore.begin!
+        yield
+      ensure
+        RequestStore.end!
+        RequestStore.clear!
+      end
+    end
+  end
+end