application_controller.rb

class ApplicationController < ActionController::Base
  # so all actions get some basic logging
  # this also makes the :log_metrics_for class macro method available to all controllers
  include MetricLoggable
end

metric_loggable.rb

module MetricLoggable
  extend ActiveSupport::Concern

  included do
    after_filter :log_action_metrics
  end

private

  def log_action_metrics
    MyLogger.write(action_metrics)
  end

  def action_metrics
    # We use instance_eval so `self` in the block is the current controller instance.
    # That way, instance variables can be used.
    base_metrics.merge instance_eval(&self.class.metric_blocks[action])
  end

  def base_metrics
    { :user_id => current_user.id, :shard => ActiveRecord::Shard.current }
  end

  module ClassMethods
    def metric_blocks
      @metric_blocks ||= Hash.new do |hash, action|
        # default: return an empty hash
        hash[action] = lambda { {} }
      end
    end

    def log_metrics_for(*actions, &block)
      actions.each do |action|
        metric_blocks[action] = block
      end
    end
  end
end

my_controller.rb

class MyController < ApplicationController
  log_metrics_for :show, :edit do
    # the block must return a hash
    { :widget_count => @widgets.size }
  end
end