Note that the 's ends up having a completely different meaning in your two sentences:
a) It's [it has] been so long since we had a meeting that I've forgotten.
b) It's [it is] so long since we had a meeting that I've forgotten.
To my ears, option a) sounds better: you are using the present perfect has been to express the ongoing situation that time has been progressing without any meetings since the last meeting you had (simple past).
Option b) is grammatical, but awkward, since you're using the simple present is to refer to a time period that extends into the past, and then combining with the simple past had. Option a) is definitely the more natural-sounding choice.