By default, JustSaying won't emit any metrics as it doesn't have any information on where or how to send them. By providing an implementation of an IMessageMonitor, you can emit metrics to any source you like, such as StatsD, Prometheus, or as structured logs.
JustSaying will automatically pick up the last registered implementation of IMessageMonitor that is registered into the DI container, so all you need to do is register one:
services.AddSingleton<IMessageMonitor, MyCustomMessageMonitor>();