I think there is perhaps a little basic confusion/misunderstanding
That is pretty common as there are different places to find what you are looking for.
Kibana -> Stack Monitoring
Is only for the Curated Experience for monitoring Elastic Stack Components
Think of this as a completely dedicated UI.
- Elasticsearch
- Kibana
- Logstash
- Beats
- Integration / APM Server
No other components will show up there like Postrges as they are not part of the Elastic Stack
For other components, there are 2 places to look.
For simple Infrastructure metrics you can Look at Kibana - Observability - Infrastructure or Host
This is an OOTb Curated UI Example
Then of course there are the 100s of Dashboards and Visualization for the Beat / Agents integration
So Go to Dashboard and Search for
Hosts / System Overview
or for Postgres search for Postgres I do not have that loaded right now but assuming you setup metricbeat properly A dashboard should be there
Yup a little confusing but hopefully that helps a bit.