Akka.Streams: add monitoring stages to measure flow throughput / graph execution time

[Aaronontheweb]

Without getting overly prescriptive on the specs for this:

1. Want one flow stage which measures the rate at which stream items are flowing through it and periodically emits that value to an external Sink of some kind. That Sink can be a Sink.ActorRef or whatever else, but the idea being that this stream can observe the quantity of messages / second passing through it and periodically emit that counter value out to the Sink. This can be used explicitly for monitoring / observability purposes or it can be used for flow-control, i.e. observing how close a system is performing compared to some internal or external resource limit and pre-emptively throttling.
2. This one is more complicated - need a way of observing how long it takes a specific "tracer round" or a user-defined message to pass from one point of the stream to another. This is designed to measure per-message latency as a gauge or histogram - so in the event that a stream begins rapidly backpressuring, that value can be observed in an external monitoring system or perhaps an alert can be fired.

We are planning on building some Phobos-specific implementations of these, but given the value they can bring outside of observability (flow control, adaptive backoff, etc) we thought they merited inclusion into the Akka.Streams core library itself.

[Aaronontheweb]

closed via #5357 - that should help quite a bit.