Singleton but as convenient as class methods; it's both.

singleton_with_class_delegator.rb

module SingletonWithClassDelegator
  extend ActiveSupport::Concern

  included do
    include Singleton
    original_delegate_method = method(:delegate).unbind if self.respond_to?(:delegate)
    extend SingleForwardable
    if original_delegate_method.present?
      # restore the delegate of ActiveSupport that has been overshadowed by SingleForwardable
      define_singleton_method :delegate, original_delegate_method
    end
  end

  class_methods do

    # delegates methods to the singleton instance
    def method_missing(method_name, *args, &block)
      if instance.respond_to?(method_name)
        def_single_delegator :instance, method_name
        public_send(method_name, *args, &block)
      else
        super
      end
    end

    def respond_to_missing?(method_name, *_args)
      instance.respond_to?(method_name) || super
    end

  end

end