By pushing feature flagging metadata into New Relic, you can understand the impact of a feature change across any metric you care about, in real time.
New Trends in Application Monitoring Systems from Velocity 2016
Anomaly detection in application monitoring systems (APM) is the gold standard. The idea is that if your APM can tell you when something is wrong, you can multiply the effectiveness of your site reliability team. Unnecessary alerts are never thrown and humans are not stuck watching dashboards.