Update prometheus client gem to fix problems with PID handling following

Gemfile

@@ -286,7 +286,7 @@ group :metrics do
   gem 'influxdb', '~> 0.2', require: false
 
   # Prometheus
-  gem 'prometheus-client-mmap', '~>0.7.0.beta9'
+  gem 'prometheus-client-mmap', '~>0.7.0.beta11'
   gem 'raindrops', '~> 0.18'
 end
 

Gemfile.lock

@@ -595,7 +595,7 @@ GEM
     premailer-rails (1.9.7)
       actionmailer (>= 3, < 6)
       premailer (~> 1.7, >= 1.7.9)
-    prometheus-client-mmap (0.7.0.beta10)
+    prometheus-client-mmap (0.7.0.beta11)
       mmap2 (~> 2.2, >= 2.2.7)
     pry (0.10.4)
       coderay (~> 1.1.0)
@@ -1047,7 +1047,7 @@ DEPENDENCIES
   pg (~> 0.18.2)
   poltergeist (~> 1.9.0)
   premailer-rails (~> 1.9.7)
-  prometheus-client-mmap (~> 0.7.0.beta9)
+  prometheus-client-mmap (~> 0.7.0.beta11)
   pry-byebug (~> 3.4.1)
   pry-rails (~> 0.3.4)
   rack-attack (~> 4.4.1)

changelogs/unreleased/pawel-prometheus_client_pid_reuse_error.yml

+---
+title: Fix Prometheus client PID reuse bug
+merge_request: 13130
+author:

config/unicorn.rb.example

@@ -121,6 +121,10 @@ after_fork do |server, worker|
   defined?(ActiveRecord::Base) and
     ActiveRecord::Base.establish_connection
 
+  # reset prometheus client, this will cause any opened metrics files to be closed
+  defined?(::Prometheus::Client.reinitialize_on_pid_change) &&
+    Prometheus::Client.reinitialize_on_pid_change
+
   # if preload_app is true, then you may also want to check and
   # restart any other shared sockets/descriptors such as Memcached,
   # and Redis.  TokyoCabinet file handles are safe to reuse