class Google::Auth::DefaultCredentials

def self.make_creds options = {}

loading the json.
override CredentialsLoader#make_creds to use the class determined by 
def self.make_creds options = {}
  json_key_io = options[:json_key_io]
  if json_key_io
    json_key, clz = determine_creds_class json_key_io
    io = StringIO.new MultiJson.dump(json_key)
    clz.make_creds options.merge(json_key_io: io)
  else
    clz = read_creds
    clz.make_creds options
  end
end