voodoo.rb

Module.class_eval do
  def redefine_instance_method(name, &block)
    old_instance_method = instance_method(name)
    define_method(name) do
      block.call(self, old_instance_method)
    end
  end

  def redefine_class_method(name, &block)
    old_method = method(name)
    metaclass = class << self; self; end

    metaclass.send(:define_method, name) do
      block.call(old_method)
    end
  end
end

class Bla
  def self.ola
    puts "Olá!"
  end

  def hello
    puts "Hello!"
  end
end

Bla.redefine_instance_method(:hello) do |instance, old_method|
  puts "instrumented"
  old_method.bind(instance).call
end

Bla.redefine_class_method(:ola) do |old_method|
  puts "yes it is instrumented"
  old_method.call
end

b = Bla.new
b.hello

Bla.ola