Nagios minimum folder size plugin
For nrpe you will need to modify nrpe.cfg to check the correct folder etc
Example nrpe.cfg line (will warn under 25GB and be critical under 20GB): command[check_folder_min_size]=/usr/local/nagios/libexec/check_folder_min_size -f /opt/solr/db/ -s g -w 25 -c 20