Using region as it works for me.

cloudwatch_test.rb

require 'pp'
require 'aws-sdk'
require 'logger'

module RAWS
  class Metrics
    def initialize
      config_path = '.riotaws.yml'
      AWS.config({
        :access_key_id => 'A...',
        :secret_access_key => 'Xx...',
        :logger    => Logger.new(STDOUT),
        :log_level => :debug,
        :region => 'us-west-2'
        #:endpoint  => 'monitoring.us-west-2.amazonaws.com'
      })
      cloudwatch_metric
    end
    def cloudwatch_metric
      cw = AWS::CloudWatch.new(:region => 'us-west-2')

      ["AWS/ELB"].each { |namespace|
        pp "#{namespace} has #{cw.client.list_metrics({:namespace => namespace}).data[:metrics].length} of metrics"
        puts "#{cw.client.list_metrics({:namespace => namespace}).data[:metrics].inspect}"
        puts "#{cw.client.list_metrics({:namespace => namespace}).inspect}"
      }
    end
  end
end

RAWS::Metrics.new