A few bits of Ruby code for using ActiveSupport::Notifications to pass custom & rails provided metrics to riemann.

activesupport instrumentation riemann.rb

# in config/initializers/riemann.rb
$riemann = Riemann::Client.new
channels_to_subscribe = ["auth","process_action.action_controller","deliver.action_mailer"]
channels_to_subscribe.each do |channel|
  ActiveSupport::Notifications.subscribe channel do |*args|
    event = ActiveSupport::Notifications::Event.new(*args)
    Rails.logger.debug "ActiveSupport::Notifications #{event.name}, #{event.payload.inspect}"
    riemann_event = {
      host: `hostname`,
      state: event.payload[:state],
      metric: (event.duration || event.payload[:metric].to_i),
      ttl: 10, # or what ever TTL you want on your metrics
      # you need to dup or you'll see weird things in tests if you expect tags to be ['sign_in','local'] then add a rails env or something in your initializer
      tags: (event.payload[:tags].dup << Rails.env),
      service: 'some-awesome-service'
    }
    $riemann << riemann_event
  end
end

# then in your code..

# instrument
ActiveSupport::Notifications.instrument("auth", :tags => ['sign_in','local'], :state => 'ok', :remote_ip => request.remote_ip, :metric => 99)

# measure
ActiveSupport::Notifications.instrument("auth", :tags => ['sign_in','local'], :state => 'ok', :remote_ip => request.remote_ip) do
  # your code
  # should populate event.duration in the subscription block
end

# some useful rails ones listed on the edge guides for RoR
# http://edgeguides.rubyonrails.org/active_support_instrumentation.html

# Highlights include 
"process_action.action_controller"
{
  controller: "PostsController",
  action: "index",
  params: {"action" => "index", "controller" => "posts"},
  format: :html,
  method: "GET",
  path: "/posts",
  status: 200,
  view_runtime: 46.848,
  db_runtime: 0.157
}

"deliver.action_mailer"
{
  mailer: "Notification",
  message_id: "[email protected]",
  subject: "Rails Guides",
  to: ["[email protected]", "[email protected]"],
  from: ["[email protected]"],
  date: Sat, 10 Mar 2012 14:18:09 +0100,
  mail: "..." # omitted for brevity
}

# for the the activesupport ones you might want to do something like:
ActiveSupport::Notifications.subscribe "process_action.action_controller" do |*args|
  event = ActiveSupport::Notifications::Event.new(*args)
  tags = [event.payload[:controller], event.payload[:action]]
  client.gauge (tags.dup << 'http_status'), 'ok', event.payload[:status]
  client.gauge (tags.dup << 'view_runtime'), 'ok', event.payload[:view_runtime]
  client.gauge (tags.dup << 'db_runtime'), 'ok', event.payload[:db_runtime]
end

I've tidied this up somewhat by creating a riemann client wrapper so i can do

# in the ActiveSupport::Notifications.subscribe block
client.gauge event.payload[:tags], event.payload[:state], 1

and populate things like hostname etc in the wrapper client.