Function monitor 

pub fn monitor<T, F>(src: Source<T>, on_each: F) -> Source<T>
where
    T: Send + 'static,
    F: FnMut(&T) + Send + 'static,

monitor(src, on_each) — invoke on_each(&item) for every element flowing through, without consuming or transforming it. Useful for telemetry instrumentation.

Akka.NET: Source.Monitor.