Tell me an approach to design a system (from a Data Science perspective) to alert a user that some value you monitor on one of his server is abnormal / deserve action (anomaly detection on time series) ?

Interview question asked to Data Scientists interviewing at Yext, Mapbox, New Relic and others: Tell me an approach to design a system (from a Data Science perspective) to alert a user that some value you monitor on one of his server is abnormal / deserve action (anomaly detection on time series) ?.