Added openVPN exporter

[Fadi-hamwi]

OpenVPN Metrics Exporter where I expose per-client traffic, connection times, and server status metrics from OpenVPN's management interface. not the status file for real-time monitoring.
OpenVPN exporter Repo

[juliusv]

Hi, the whole docs site and its structure just got changed with the launch of the new prometheus.io website yesterday - the same doc now lives at https://github.com/prometheus/docs/blob/main/docs/instrumenting/exporters.md, could you rebase the PR and make the change against that file instead?

I would also note that the exporters list is (mostly, although sadly not perfectly) alphabetically sorted, so it would be nice to sort the new entry in alphabetically.

Regarding the metrics that the exporter produces:

• openvpn_client_bytes_received and openvpn_client_bytes_sent should have a _total suffix since they are counters, see https://prometheus.io/docs/practices/naming/.
• I think the connection_time label contains the "connected since" timestamp, right? So you have that both as a label on all metrics, and as a separate metric? Just wondering whether it'd be better to omit that on the individual metrics and only join it in when needed.