implement Log.withMdc for creating Log with MDC preset

[dfakhritdinov]

No description provided.

[mikost]

Edit: Oh, now I see - the withMDC on line 144 is an internal implementation detail, not available to the user of the class.

Original comment:
On line 144 in the changed file, Log.scala, there was already a similar, albeit slightly different, method:

    def withMDC(mdc: Log.Mdc)(log: => Unit): Unit = {
      import Mdc.MdcOps
      mdc.context match {
        case None => log
        case Some(mdc) =>
          val backup = MDC.getCopyOfContextMap
          MDC.clear()
          mdc.toSortedMap foreach { case (k, v) => MDC.put(k, v) }
          log
          if (backup == null) MDC.clear() else MDC.setContextMap(backup)
      }
    }

I can't immediately discern whether that's for the same use case (as was catered for in this MR) or another one. Can you elaborate, please? Perhaps some ScalaDoc-ing is warranted for these two? Or one of them should be deprecated?