Nagios is a monitoring platform.
NRPE is an addon that allows you to execute plugins remotely on Linux/Unix hosts.
Nagios check utilit may be used by collector. Example : Telegrah
Plugin always print something to STDOUT that tells if the service is working or why it is failing.
/usr/local/nagios/libexec/check_load -w 5,6,7 -c 7,8,9
OK - load average: 0.49, 0.24, 0.15|load1=0.490;5.000;7.000;0; load5=0.240;6.000;8.000;0; load15=0.150;7.000;9.000;0;
The output structure is described in the plugin API section
'label'=value[UOM];[warn];[crit];[min];[max]
check_load -help