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require 'yaml'
node = YAML.load_file("/cs/etc/node_metadata.yml")

Facter.add('cs_nat_ip') do
  setcode do
    node[:nat_ip_addr]
  end
end

in irb get this: 

=> #, @suitable=true, @name="cs_nat_ip", @confines=[], @code=#, @value=nil>], @value=nil>