your vet is crazy if he thinks it's normal for a dog's anal glands to leak pus.
Pus is a sign of infection. Normally, most dogs never ever have anal gland issues, once they start, you can usually count on it being an ongoing thing. Not good
Constant impacted/infected anal glands can result in Perianal fistula's also not good.
I would read this site perianal-fistulas
join their email list which is a wealth of information/suggestions/advice and support
I would be looking for another vet as well