Merge branch '33125-fix-sidekiq-prometheus' into 'master'

Fix for sidekiq prometheus files being wiped accidentally

Closes #33125

See merge request gitlab-org/gitlab!18568

config/initializers/7_prometheus_metrics.rb

@@ -34,6 +34,12 @@ Sidekiq.configure_server do |config|
   config.on(:startup) do
     # webserver metrics are cleaned up in config.ru: `warmup` block
     Prometheus::CleanupMultiprocDirService.new.execute
+    # In production, sidekiq is run in a multi-process setup where processes might interfere
+    # with each other cleaning up and reinitializing prometheus database files, which is why
+    # we're re-doing the work every time here.
+    # A cleaner solution would be to run the cleanup pre-fork, and the initialization once
+    # after all workers have forked, but I don't know how at this point.
+    ::Prometheus::Client.reinitialize_on_pid_change(force: true)
 
     Gitlab::Metrics::Exporter::SidekiqExporter.instance.start
   end