Add link to prometheus version of expression.

~~when you have a chance, check more complex queries with me? they don't run when linking, but also when I straight up enter them in prometheus.~~
I was searching for servers by axis_id, instead of server_id -.-

e.g. sum(rate(voigtkampff_signup_decisions_total[10m])) by (is_blacklisted,is_classified_spam) on tss_vk

requested in #210 
Ah! The expressions needs to get any variables interpolated before being linkified. Meh, I guess we need to pass in the variables into the expression as well?