adding piwik analytics to gitlab.rb

What's the right syntax for adding piwik to the config?

gitlab_rails['piwik_url'] = "http://domain.com"

gitlab_rails['piwik_site_id'] = 1

or

extra['piwik_url'] = "http://domain.com"

extra['piwik_site_id'] = 1

none of them seems to work