Additional prom CounterValue metric for peer flaps

[bartdorlandt]

Describe the feature
Besides having the stateDesc providing the exact peer state at a moment in time, I'd like to see a metric (type CounterValue) that is increased every time a BGP peer flaps. Allowing to see increases also in between prometheus scrapes.

Use cases / additional context
The stateDesc (file: bio-routing/bio-rd/metrics/bgp/adapter/prom/bgp_prom_adapter.go) will provide the current state at the moment of scraping. Most probably providing it with a value of 0 or 6 (Down or Established).
Though flaps in between those state are only see if the flap takes longer than the prometheus scrape time.

A flap down would add 1 to the counter as well as a flap up.

The metric would need the label for the peer IP as well, to show which peer has flapped. Probably along with the local AS, peer AS and VRF as it is currently done in the collectForPeer().