Wrap methods in rack-mini-profiler steps

mini_profiler_instrumenter.rb

# Usage:
# class MyClass
#   extend MiniProfilerInstrumenter
#  
#   def my_slow_method
#     ...
#   end
#   instrument_method :my_slow_method

module MiniProfilerInstrumenter
  def instrument_method method_name
    alias_method "#{method_name}_without_instrument", method_name
    define_method "#{method_name}_with_instrument" do |*args|
      Rack::MiniProfiler.step("#{self.class}##{method_name}") do
        send("#{method_name}_without_instrument", *args)
      end
    end
    alias_method method_name, "#{method_name}_with_instrument"
  end
end