gistfile1.rb

# this is the plugin lib
module ActsAsTraceable
  
  class TracedClass
    def initialize(object)
      @object = object
    end
    def method_missing(method, *args, &block)
      puts "[TRACE] START: #{@object.class.to_s}##{method}(#{args.map { |arg| arg.inspect }.join(', ')})"
      @object.send(method, *args, &block)
      puts "[TRACE] END: #{@object.class.to_s}##{method}"
    end
  end
  
  def acts_as_traceable
    class << self
      alias_method :__new__, :new
      def new(*args)
        TracedClass.new(self.__new__(*args))
      end
    end
  end
end

# this is the plugin init
class << Object
  include ActsAsTraceable
end

# this is your class, just add acts_as_Traceable
class Foo
  acts_as_traceable

  def fred(foo, bar)
    puts "before yield"
    yield
    puts "after yield"
  end
end

# testing
Foo.new.fred("foo", Object.new) do
  puts "during yield"
end