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)
nginx.net.current is the same as nginx.net.connections
Merge pull request #665 from dcrosta/nginx-check-total-connections

add two new metrics to nginx check