node_exporter was listening on 0.0.0.0:9100 on helsinki-a and london-a,
exposing metrics to the public internet.
Changes:
- Add node_exporter_bind_tailscale flag (default false) to opt in
- Set flag on helsinki-a and london-a host_vars
- Debian: configure ARGS in /etc/default/prometheus-node-exporter
- FreeBSD: use native node_exporter_listen_address rc.conf variable
- Add handlers to restart on config change
Prometheus already scrapes via Tailscale IPs, no scrape config changes needed.
Fixes PESO-98
