feature request: return bgp session_status instead of session_up

[33Fraise33]

Hello,

I am trying to set alerting rules for our datacenter coreswitches with this exporter.
i was planning to use junos_bgp_session_up but this is 1 if up and 0 if everything else. This unfortunately does also show as "failed" when the session is in admin down.

The proposal is to create a new metric called junos_bgp_session_state which then return values for the possible states (idle, active, established, shutdown).

Another option is to create an entry like exists for interfaces: junos_bgp_session_error_status which then returns 1 if operational and admin state differ.