Add metric for the number of external authentication requests

We've hit issues before where, due to issues with ns_server/LDAP, memcached would end up with a large number of pending external authentications.

External authentication is managed by ExternalAuthManagerThread. ns_server will establish a connection to memcached and offer to be an external auth provider. Then, any authentications which cannot be done locally will be proxied to ns_server, and ns_server will speak to LDAP.

The pending responses are in ExternalAuthManagerThread::requestMap. We should expose Prometheus metrics with the number of entries added and removed from that map, to be able to more easily detect when we're waiting on ns_server.