Be sure to install the datadog agent on your server before applying this code to your app. Instructions are at https://app.datadoghq.com/account/settings#agent or use my Sunzi recipe at https://github.com/Shift81/sunzi-recipes/blob/master/ubuntu/precise/datadog.sh

Gemfile

gem 'dogstatsd-ruby'

instrumentation.rb

require 'statsd'

$statsd = Statsd.new

ActiveSupport::Notifications.subscribe /process_action.action_controller/ do |*args| 
  event = ActiveSupport::Notifications::Event.new(*args)
  controller = "controller:#{event.payload[:controller]}"
  action = "action:#{event.payload[:action]}"
  format = "format:#{event.payload[:format] || 'all'}"
  format = "format:all" if format == "format:*/*" 
  host = "host:#{ENV['INSTRUMENTATION_HOSTNAME']}"
  status = event.payload[:status]
  tags = [controller, action, format, host] 
  ActiveSupport::Notifications.instrument :performance, :action => :timing, :tags => tags, :measurement => "request.total_duration", :value => event.duration
  ActiveSupport::Notifications.instrument :performance, :action => :timing, :tags => tags,  :measurement => "database.query.time", :value => event.payload[:db_runtime]
  ActiveSupport::Notifications.instrument :performance, :action => :timing, :tags => tags,  :measurement => "web.view.time", :value => event.payload[:view_runtime]
  ActiveSupport::Notifications.instrument :performance, :tags => tags,  :measurement => "request.status.#{status}" 
end

def send_event_to_statsd(name, payload)
  action = payload[:action] || :increment
  measurement = payload[:measurement]
  value = payload[:value]
  tags = payload[:tags]
  key_name = "#{name.to_s.capitalize}.#{measurement}"
  if action == :increment
    $statsd.increment key_name, :tags => tags
  else
    $statsd.histogram key_name, value, :tags => tags
  end
end

ActiveSupport::Notifications.subscribe /performance/ do |name, start, finish, id, payload| 
  send_event_to_statsd(name, payload) if Rails.env == 'production'
end