This endpoint accepts custom parameters which are then stored on the metadata field of the session. This can then be used for filters and to define audiences in the application, or rendered into an email. Also, any key prefaced by "customer_" will be added to the customer metadata field once the session has been identified. All metadata should be stored as text.
This command will result in a session with metadata of {'customer_is_vip': 'yes'} and customer metadata of {'is_vip': 'yes'}.
_rejoiner.push(['setSessionMetadata', {
'customer_is_vip': 'yes',
}]);