http://thenewstack.io/monitoring-101-collecting-right-data/
Metrics capture a value pertaining to your systems at a specific point in time.
Metrics are usually collected once per second, one per minute, or at another regular interval to monitor a system over time.
For different Metric Data Types and the metric names they produce see https://docs.datadoghq.com/developers/metrics/types/?tab=histogram#metric-types