Monitoring Nginx with Munin
Munin is a free software networking and infrastructure monitoring system. We chose it because the configuration is simple and allows us to demonstrate the principles. You will certainly prefer to use the same monitoring solution that you use for your other needs. If you do not currently use anything that is not very probable, Munin is as good an option as any other to start with.
Munin was developed with easy extensibility via plugins in mind. It uses the famous RRDTool time series database that was first developed as part of MRTG but then found its way to many other free and open source software as the time series database and time-based graphs engine.
Installing Munin is not as easy as we wish it were but the process is thoroughly described in the online Munin guide http://guide.munin-monitoring.org/en/latest/installation/index.html. There is also a book about Munin published by Packt Publishing, Instant Munin Plugin Starter. It may be a little old but still...