Slf4j v2 backend

[justcoon]

depends on #638

fixes: #637

similar like slf4j v1 implementation (for easier migrations), but using some of new features like key value pairs
instead of MDC which have issue (like #491), as MDC using ThreadLocal

throwable/exception handling was changed to:

• when there is Cause.Die or Cause.Fail with Throwable, that throwable is logged
• otherwise cause is transformed to zio.FiberFailure - as it was originally in all cases

TODO:

• markers
  • check support for multiple Markers - slf4j v1 backend is able to set just one marker, by marker name
  • check other marker related features - option to provide created Marker?

[jdegoes]

Matching on a cause directly is not reliable because of nodes like or / and / stackless /e tc.

Instead, if you are trying to just "get the most important throwable", then do something like squash or defects or perhaps: (cause.errors.collect { case t : Throwable } ++ cause.defects).head (or its equivalent).

Because you want to prefer an error before you look into the defects, and you want to be robust in the presence of intermediate / metadata nodes.

[justcoon]

@jdegoes

I changed that to

        if (!cause.isEmpty) {
          val maybeThrowable = (cause.failures.collect { case t: Throwable => t } ++ cause.defects).headOption
          maybeThrowable match {
            case Some(t) => throwable = t
            case None    => throwable = FiberFailure(cause)
          }
        }

can you check it? Thank you

[jdegoes]

One quick comment to make extracting a Throwable from a Cause more robust.

[jdegoes]

Looks great! Please also open a ticket on ZIO to add Cause#toThrowable that does something similar (but more efficiently).