BGP messages: only iBGP should include a LocalPref

[vaizki]

Andrey on Kubernetes Slack reported:

Hi again, just checked juniper logs regarding bgp sessions with metallb speakers and found something unexpected: speakers are sending localpref attribute to eBGP peers
BGP_ERROR_UNEXPECTED_LOCAL_PREF: bgp_recv_v4_update: external peer x.x.x.x (External AS 65400) UPDATE included LOCALPREF attribute

Indeed, only iBGP sessions inside the same AS can contain a localpref but right now it is included in all advertisements.

https://github.com/google/metallb/blob/d38ad1ef80188b6e5bcdb246cff72039d6d52e3a/internal/bgp/messages.go#L357

Probably doesn't break anything but may flood some logs.