This enables users of the bio-rd library to reuse the state numbering.
Otherwise users have to maintain their own constats to avoid magic
numbers.

* first steps to metrics

* next small step

* fixed tests

* added comments

* postponed RouterAccepted and RoutesRejected to next step

* read update counters

* added test

* added missing test file

* Route not used any more

* added metrics for session state and time since establish

* increased test coverage, time since establish impl.

* added prom collector

* added metrics endpoint to BGP example

* added metrics for VRF

* removed commented metrics

* fixed logging inconsistency in example

* Update main.go

* gofmt

* fixed typo