Fix node.statusQuery colliding legends: multi-neighbor devices previously
sourced from raw per-neighbor BGP session-state series that collide under
one legend, so an arbitrary neighbor won. Replace with a combined
device-up * worst-BGP-session query per device, with an explicit fallback
so zero-BGP devices (access switches) read as neutral/OK.
Add site label via Prometheus relabel (derived from device naming
convention) instead of per-query label_replace(). Add site/device template
variables, BGP sessions table, ports/interfaces table (Grafana merge
transform, not PromQL join), and per-site throughput graphs.
Refs #49
Add prometheus node to the Containerlab topology, scraping the gnmic
exporter (172.16.0.70:9273) at 5s interval with relabeling to clean
device/interface/neighbor_address/vlan/mac_address/afi_safi labels.
Validated against BGP and system metrics confirmed in #44. Existing
external Prometheus instance stays untouched in parallel.
Refs #45
