Remove unused StatsD code and expose StatsD as a global variable (#11232)

The instrumentation code was used for StatsD metrics collection
prior to the switch to the nsa gem and should have been removed
at that point as it no longer does anything at all

config/initializers/instrumentation.rb

-# frozen_string_literal: true
-
-instrumentation_hostname = ENV.fetch('INSTRUMENTATION_HOSTNAME') { 'localhost' }
-
-ActiveSupport::Notifications.subscribe(/process_action.action_controller/) do |*args|
-  event      = ActiveSupport::Notifications::Event.new(*args)
-  controller = event.payload[:controller]
-  action     = event.payload[:action]
-  format     = event.payload[:format] || 'all'
-  format     = 'all' if format == '*/*'
-  status     = event.payload[:status]
-  key        = "#{controller}.#{action}.#{format}.#{instrumentation_hostname}"
-
-  ActiveSupport::Notifications.instrument :performance, action: :measure, measurement: "#{key}.total_duration", value: event.duration
-  ActiveSupport::Notifications.instrument :performance, action: :measure, measurement: "#{key}.db_time", value: event.payload[:db_runtime]
-  ActiveSupport::Notifications.instrument :performance, action: :measure, measurement: "#{key}.view_time", value: event.payload[:view_runtime]
-  ActiveSupport::Notifications.instrument :performance, measurement: "#{key}.status.#{status}"
-end

config/initializers/statsd.rb

@@ -3,10 +3,10 @@
 if ENV['STATSD_ADDR'].present?
   host, port = ENV['STATSD_ADDR'].split(':')
 
-  statsd = ::Statsd.new(host, port)
-  statsd.namespace = ENV.fetch('STATSD_NAMESPACE') { ['Mastodon', Rails.env].join('.') }
+  $statsd = ::Statsd.new(host, port)
+  $statsd.namespace = ENV.fetch('STATSD_NAMESPACE') { ['Mastodon', Rails.env].join('.') }
 
-  ::NSA.inform_statsd(statsd) do |informant|
+  ::NSA.inform_statsd($statsd) do |informant|
     informant.collect(:action_controller, :web)
     informant.collect(:active_record, :db)
     informant.collect(:active_support_cache, :cache)