Add status_path label to connected_clients server metric.

[phyber]

This fixes an issue where the exporter would fail to work if a user attempted to monitor multiple OpenVPN processes in server mode as the metrics would clash without the status_path label. This resulted in the exporter failing and returning a HTTP 500.

Before the fix:

# curl -I localhost:9176/metrics
HTTP/1.1 500 Internal Server Error
Content-Type: text/plain; charset=utf-8
X-Content-Type-Options: nosniff
Date: Tue, 01 May 2018 11:06:28 GMT
Content-Length: 182

# curl localhost:9176/metrics
An error has occurred during metrics gathering:

collected metric openvpn_openvpn_server_connected_clients gauge:<value:2 >  was collected before with the same name and label values

After fix:

# curl localhost:9176/metrics 2>/dev/null | grep connected_clients
# HELP openvpn_openvpn_server_connected_clients Number Of Connected Clients
# TYPE openvpn_openvpn_server_connected_clients gauge
openvpn_openvpn_server_connected_clients{status_path="/var/log/openvpn-status-server.log"} 1
openvpn_openvpn_server_connected_clients{status_path="/var/log/openvpn-status-server2.log"} 1

[BartVerc]

Thanks for the PR! Confirmed the bug. Tested and merged.