We have graylog2 set up to monitor the syslogs, etc. However we're wondering if we can use it to monitor the different services, if they're down or not, if the hard disk is filling up.
Does it provide such features? Or will we have to install icinga2 along side it?
Graylog is a log management solution, not a monitoring tool. It can't do any active checks like Nagios/Icinga.
I guess you could abuse(!) it for such a purpose in a limited fashion if you use the trigger and alert functionality - just send out log lines stating things you want to watch out for and then define a trigger that generates an alert. As I said, this will be limited and no replacement for a full featured monitoring solution.
