I monitor the engagement indicators by checking the number of posts and the popularity of the topics. They guide me in knowing what my audience identifies with.
I look at views and more importantly what area they are viewing the most. I definitely look at engagement rates, as well. I look at referrals and where people are visiting from.
I rarely look at the forum's built-in metrics because they're just for vanity and really are not that helpful.
I think metrics on engagement and retention matters most. You should be tracking active users, thread participation, response times, and return visits.