A type of metric that has a value that always increases when it changes, except when it is reset to zero. Automobile odometers are a simple counter metric example. Odometers indicate the number of miles that a car has been driven. Odometer values never go down, except when they are reset to zero.
There are two types of counter metrics: periodic and accumulating.
Periodic counter metrics are reset to zero each time the client sends a measurement to the server. This means that each data point for a periodic counter metric is independent of the other data points belonging to that metric.
Accumulating counter metrics are only reset to zero when the service is reset. Each new value is added to the last one. You can compare two measurements to get the rate of value accumulation.
For more information
In the Metrics manual: