Warn on finished span in runWithSpan

[SimunKaracic]

[ivantopo]

Given that this issue is related to the comments on https://github.com/kamon-io/Kamon/issues/913, I wouldn't limit the warnings to runWithSpan. There are many ways we could be interacting with finished Spans, so I would rather put these things on the tag methods on the Span class itself.

And this opens two questions:

• What do we do with Empty and Remote spans. I'm guessing they should also get warning.
• Should the warning be there all the time or should we have a specific flag for it? I'm inclined to believe this should be behind some sort of debug flag.

[SimunKaracic]

Added flag, and a bunch of log messages everywhere. Not sure if this is how you wanted it to look @ivantopo , so please check it out and let me know if you like it