code: def check_subscriber_key_length(app_configs=None, **kwargs):
  	
  	from . import settings as djstripe_settings
  
  	messages = []
  
  	key = djstripe_settings.SUBSCRIBER_CUSTOMER_KEY
  	key_size = len(str(key))
  	if key and key_size > 40:
  		messages.append(
  			checks.Error(
  				"DJSTRIPE_SUBSCRIBER_CUSTOMER_KEY must be no more than 40 characters long",
  				hint="Current value: %r (%i characters)" % (key, key_size),
  				id="djstripe.E001",
  			)
  		)
  
  	return messages
docs: Check that DJSTRIPE_SUBSCRIBER_CUSTOMER_KEY fits in metadata.
  
  	Docs: https://stripe.com/docs/api#metadata