Dead simple Rails monitoring

We are working on a Rails application which processes highly sensitive information. This system can not be accessed outside of a VPN. We try to minimize dependencies like including gems and we do not use any external services.The application has grown for a few years now and performance was never really an issue. The virtual machine hummed away with single digit cpu percentages. Yet, sometimes we saw spikes that worried us. Who was the culprit? A delayed job? A long running request? We had to introduce some sort of monitoring…Due to the sensitive nature of the data, we decided not to go for one of the standard monitoring products. We brewed our own solution and it was surprisingly simple and rewarding.