Client metrics cover the traffic between the client and the Varnish cache.
client_req to keep track of traffic volume - is it increasing or decreasing, is it spiking etc. Sudden changes might indicate problems.
sess_dropped to see if the cache is dropping any sessions. If so you might need to increase
varnishstat -1 | grep "sess_conn\|client_req \|sess_dropped" MAIN.sess_conn 62449574 3.38 Sessions accepted MAIN.client_req 184697229 9.99 Good client requests received MAIN.sess_dropped 0 0.00 Sessions dropped for thread