title: TPL Dataflow log item if send to ITargetblock
body: <p>I'm actually facing a problem with TPL dataflow (it seems that an item is added twice). Therefore I wanted to know if there is the option to inject a callback, which I can use for logging, if an item is posted to a target block. I checked the <code>ActionBlock</code>, <code>BufferBlock</code> constructors, and also the <code>LinkTo</code> methods but didn't find anything.</p>
  
  <p>Any hints?</p>