Run before_fork only once on boot

This aligns behavior of before_fork with Puma where before_fork
is called only on master boot, also none of our code needs to be run
before each worker fork, instead we have a use-case for deleting
Prometheus files only on boot time.

Related https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29788

changelogs/unreleased/puma-restart-master.yml

+---
+title: Run before_fork only once on boot for Unicorn.
+merge_request:
+author:
+type: changed

files/gitlab-cookbooks/gitlab/templates/default/unicorn.rb.erb

@@ -33,16 +33,18 @@ run_once = true
 
 before_fork do |server, worker|
   if run_once
+    # There is a difference between Puma and Unicorn:
+    # - Puma calls before_fork once when booting up master process
+    # - Unicorn runs before_fork whenever new work is spawned
+    # To unify this behavior we call before_fork only once (we use
+    # this callback for deleting Prometheus files so for our purposes
+    # it makes sense to align behavior with Puma)
     run_once = false
 
-    # The Unicorn before_exec hook only gets called on a Unicorn
-    # restart, but we want to clean things up (e.g. metric files) at
-    # first boot.
-    Gitlab::Cluster::LifecycleEvents.do_master_restart
+    # Signal application hooks that we're about to fork
+    Gitlab::Cluster::LifecycleEvents.do_before_fork
   end
 
-  Gitlab::Cluster::LifecycleEvents.do_before_fork
-
   old_pid = "#{server.config[:pid]}.oldbin"
   if old_pid != server.pid
     begin