Metrics from VICI client

[Crevil]

This change introduces prometheus metrics from a strongswan process.
It collects statistics by inspecting the results of swanctl --list-conn and swanctl --list-sas calls.

Package vici is ported from https://github.com/bronze1man/goStrongswanVici and updated with error handling and removal of fmt.Print calls.

Package strongswan implements the actually collection of data and reports to the PrometheusReporter instance.

[eXeDK]

This is pretty cool 👍 💯

[emilingerslev]

Good work. Had a bit hard time of getting a full overview of the code, but it seems pretty solid 👍
I've added my proposals for the buckets

[Crevil]

I've not set any of all the labels we discussed @emilingerslev . And I've removed the notes we made when discussing it, so maybe we should give it another try?
Currently I only have child SA names on the metrics relevant for that.

[emilingerslev]

Alright, so yeah maybe we should go over the labels again. From the top of my head I got for Child SA's

• Local IP range
• Remote IP range

For IKE SA's (and the Child SA's below):

• Local Peer IP
• Remote Peer IP

[Crevil]

I'll add that. Then we can get it merged and out and running. Then we can add new labels later on.

[Crevil]

I've added labels. An example metric is now like this.

strong_duckling_ike_sa_bytes_in_total{child_sa_name="net-net-0",ike_sa_name="gw-gw",instance="strongswan1:8000",job="strongswan1",local_ip_range="10.101.0.1/32",local_peer_ip="172.22.0.2",remote_ip_range="10.102.0.1/32",remote_peer_ip="172.22.0.3"}

[Crevil]

@emilingerslev ready for another review of the changes.

[emilingerslev]

Looks good 👍 lets merge this! 💪

[Crevil]

Great. I'll work on a good commit message :D