Fix for sidekiq prometheus files being wiped

Due to a race condition during sidekiq process start-up, we might end
up losing prometheus db files due to doing a cleanup after first
creating them. By re-initializing after every cleanup, we side-step
the issue, but it's not really a clean solution.

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