add two new metrics to nginx check

* nginx.net.conn_opened_per_s, number of new HTTP connections created per second
* nginx.net.current, total number of current HTTP connections, regardless of their state (sum of reading, writing, waiting)

The first one is of interest to us -- we run a service that relies on re-use of HTTP keep-alive sessions, and the cost of establishing new connections is high. The second seems like an obvious convenience, though it could be recreated in charts in DD web.