For monitoring anything, Zabbix is an excellent tool and will run on a raspberry pi.
ELK or Splunk is a bit advanced. If you are new to monitoring, I’d recommend starting off with Grafana or Zabbix. But there are other tools out there. Nagios is popular as well.
I’ve been working in the application and network monitoring area for the past 15 years, so let me know if I can help.