The units will actually depend on the metric backend used. Rate is a general interface that just has a mark method.

For the Coda Hale Metrics backend, the custom metrics Rate is backed by a Meter. Meters record rates in events per second. These per-second rates are reported as moving averages for 1 minute, 5 minute, and 15 minute windows. And there's also a mean rate for the lifetime of the application.

For the Prometheus backend, the Rate is backed by a monotonic counter.

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