Log Concentrator Graylog

Hi, in this regard Proxmox VE acts similar to a common Debian. You can use everything you'd use on those system.
By default rsyslog and systemd-journald are active logging daemons.
 
* I think in both cases you should be fine by forwarding the logs with 'rsyslog' to your graylog (AFAIR they do have a syslog receiver)
* consider using REPL if graylog supports it for reliable message delivery
* it's probably best to add a dedicated rsyslog snippet to '/etc/rsyslog.d/' to be more robust across upgrades

I hope this helps!