Akka.Streams: make LogLevel customizable on the .Log method

[Aaronontheweb]

Is your feature request related to a problem? Please describe.

The .Log method in Akka.Streams doesn't currently support a LogLevel and isn't really friendly for users to actually use.

var task = source.DivertTo(evenSink, i => i % 2 == 0)
  .Log("foo", i => $"log me {i}")
  .DivertTo(threeSink, i => i % 3 == 0)
  .DivertTo(fiveSink, i => i % 5 == 0)
  .RunWith(Sink.ForEach<int>(i => Console.WriteLine($"{i} is unhandled")), actorSystem);

Describe the solution you'd like

I think it'd be better if:

• We allowed users to just pass in a LogLevel on the given call;
• Alternatively, if we exposed the ILoggingAdapter directly as a method and just let the user call it normally, rather than doing this cute Func<T, object> junk.