Add basic support for akka streams

[yannick-cw]

What about a possibility to trace individual stream stages as in

def traceRequestForPool[T, Mat](
      connectionPool: Flow[(HttpRequest, T), (Try[HttpResponse], T), Mat],
      parentSpan: Span): Flow[(HttpRequest, T), (Try[HttpResponse], T), Mat]

but make it generic for in and output types

Additionally something saying trace a stream stage until the stream is finished would be great

[Sebruck]

good idea!