The most commonly mentioned monitoring/trending software compatible with PostgreSQL is Nagios, which is itself a popular open source project active for more than 10 years now. You can monitor just about everything -OS, network, database, and applications- with either the base Nagios or using one of its many plugins.
Nagios is primarily a monitoring system that looks for common problems and then issues alerts when they happen. You can use it to detect when network hosts have gone down, when the database is doing something unexpected, and e-mail out alerts when those conditions are met. If your requirements include things like "page people when the database disk is running out of space based on this on-call support schedule," Nagios ...
