Do not trace Akka Streams-related actors by default

[ivantopo]

When a traced request enters in contact with a Stream we end up tracing A LOT of messages like OnSubscribe, AsyncInput, RequestMore, and OnNext that really don't add any value to the trace. Those are internal implementation details that we shouldn't expose out of the box.

A good thing is that all of those spans are created by the the graph interpreter so we could just avoid creating spans from akka.stream.impl.fusing.ActorGraphInterpreter actors and that should be it.