class Google::Cloud::Credentials

def initialize keyfile, scope: nil
  verify_keyfile_provided! keyfile
  if keyfile.is_a? Signet::OAuth2::Client
    @client = keyfile
  elsif keyfile.is_a? Hash
    hash = stringify_hash_keys keyfile
    hash["scope"] ||= scope
    @client = init_client hash
  else
    verify_keyfile_exists! keyfile
    json = JSON.parse ::File.read(keyfile)
    json["scope"] ||= scope
    @client = init_client json
  end
  @client.fetch_access_token!
end