[feat] Add bgp metrics (bgp_session_info)

[jonasbadstuebner]

As described in #547, this PR adds the metrics to monitor the current state of the bgp sessions.

The port of the peer is hard coded, since it is not currently possible to change the port in kube-vip. As soon as this feature is implemented, the port will be accessible by using the peers state, similar to GetNeighborAddress().
If this should be refactored later, just tell me and I append the 179 in the peerDescription directly.

Example metrics look like this:

# HELP bgp_session_info Display state of session by setting metric for label value with current state to 1
# TYPE bgp_session_info gauge
bgp_session_info{peer="123.4.123.4:179",state="ACTIVE"} 0
bgp_session_info{peer="123.4.123.4:179",state="CONNECT"} 0
bgp_session_info{peer="123.4.123.4:179",state="ESTABLISHED"} 1
bgp_session_info{peer="123.4.123.4:179",state="IDLE"} 0
bgp_session_info{peer="123.4.123.4:179",state="OPENCONFIRM"} 0
bgp_session_info{peer="123.4.123.4:179",state="OPENSENT"} 0
bgp_session_info{peer="123.4.123.4:179",state="UNKNOWN"} 0
bgp_session_info{peer="123.4.123.5:179",state="ACTIVE"} 0
bgp_session_info{peer="123.4.123.5:179",state="CONNECT"} 0
bgp_session_info{peer="123.4.123.5:179",state="ESTABLISHED"} 1
bgp_session_info{peer="123.4.123.5:179",state="IDLE"} 0
bgp_session_info{peer="123.4.123.5:179",state="OPENCONFIRM"} 0
bgp_session_info{peer="123.4.123.5:179",state="OPENSENT"} 0
bgp_session_info{peer="123.4.123.5:179",state="UNKNOWN"} 0

[thebsdbox]

This is excellent ! (Thankyou ! )

[jonasbadstuebner]

Regarding the failing check: Is this something I broke? I think I didn't even touch these files...so I guess not?
Please let me know.

[jonasbadstuebner]

I think #557 should fix this, right? :)

[thebsdbox]

Yeah, the linter was updated recently and it's generating a load of noise in old code. 🙄

[thebsdbox]

If this is good to go, I'm happy to merge as is.

[thebsdbox]

/LGTM

[jonasbadstuebner]

From my side, it's ready to merge. If there are any issues occurring or changes wanted by others, I will propose a follow up PR.