If you prefer other tools over Nagios or Cacti, there are also proper plugins around. Depending on which tools you like, you might even have to come up with your own plugins to do your setup work as intended.
The Zabbix plugin is called pg_monz. More information about this plugin can be found at http://pg-monz.github.io/pg_monz/index-en.html. It basically provides everything you need to integrate Zabbix with PostgreSQL and fetch all of the important information.
The pganalyze-collector is a command-line tool that collects information about Postgres databases as well as queries executed on them. All of the data is converted to JSON (JavaScript Object Notation) and can be used by a tool of your choice for processing.
The module creates information about CPU, memory, RAM, tables, schemas, databases, queries, and so on.
The pg_view is another tool that can be used to monitor PostreSQL. The pg_view was developed by a major German retail company called Zalando, and it can be used free of charge. It is essentially a command-line tool, showing nicely condensed information on a simple command-line-based GUI.
More information on this tool can be found at https://github.com/zalando/pg_view.