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:
=> #<Facter:0x2ab5213ca428 @ldapname="cs_nat_ip", @tags=[], @searching=false, @name="cs_nat_ip", @resolves=[#<Facter::Resolution:0x2ab5213ca090 @fact=#<Facter:0x2ab5213ca428 ...>, @suitable=true, @name="cs_nat_ip", @confines=[], @code=#<Proc:0x00002ab5213cb008@(irb#1):12>, @value=nil>], @value=nil>