configurable.rb

module Configurable
  def config(*attrs)
    @configuration ||= Struct.new(*attrs).new
  end

  def configure
    yield(@configuration) if block_given?
  end
end

class ApiGateway
  extend Configurable
  
  config :api_key, :api_host
end

ApiGateway.configure do |config|
  config.api_key = '<API Key>'
  config.api_host = '<http://example.com>'
end

ApiGateway.config
=> #<struct api_key="<API Key>", api_host="<http://example.com>">