[zeromq-dev] Adding new zmq_getsockopt() to retrieve number of subscriptions from XPUB socket

[Francesco]

Hi zeromq team,
For "observability" / debugging I think it would be really really
useful to be able to retrieve the number of subscriptions recorded by
the 'mtrie_t' object inside a (X)PUB socket.

Would you accept a PR adding such option?

Thanks,
Francesco