Use separate Prometheus metrics dirs in dev/test

Store Sidekiq and Web server metrics from Prometheus in separate
directories in `development` and `test` environments.

config/initializers/7_prometheus_metrics.rb

 require 'prometheus/client'
 require 'prometheus/client/support/unicorn'
 
+# Keep separate directories for separate processes
+def prometheus_default_multiproc_dir
+  return unless Rails.env.development? || Rails.env.test?
+
+  if Sidekiq.server?
+    Rails.root.join('tmp/prometheus_multiproc_dir/sidekiq')
+  elsif defined?(Unicorn::Worker)
+    Rails.root.join('tmp/prometheus_multiproc_dir/unicorn')
+  elsif defined?(::Puma)
+    Rails.root.join('tmp/prometheus_multiproc_dir/puma')
+  else
+    Rails.root.join('tmp/prometheus_multiproc_dir')
+  end
+end
+
 Prometheus::Client.configure do |config|
   config.logger = Rails.logger
 
   config.initial_mmap_file_size = 4 * 1024
-  config.multiprocess_files_dir = ENV['prometheus_multiproc_dir']
 
-  if Rails.env.development? || Rails.env.test?
-    config.multiprocess_files_dir ||= Rails.root.join('tmp/prometheus_multiproc_dir')
-  end
+  config.multiprocess_files_dir = ENV['prometheus_multiproc_dir'] || prometheus_default_multiproc_dir
 
   config.pid_provider = Prometheus::Client::Support::Unicorn.method(:worker_pid_provider)
 end