title: String interpolation in logRequest and logResult in akka-http
body: <p>I am using <code>akka-http</code> + the directives <code>logRequest</code> + <code>logResult</code></p>
  
  <p>I have something along those lines</p>
  
  <pre><code>def entryPoint() = {
      val sid = UUID.randomUUID().toString
      logRequest( s" request[$sid]") {
        logResult(s"response[$sid]") {
            val sid2 = UUID.randomUUID().toString
            println(s"sid2 : $sid2")
            complete("Ok")
        }
      }
  }
  </code></pre>
  
  <p>A 1st request prints <code>sid</code> and <code>sid2</code> : </p>
  
  <pre><code>sid  = 87c7db3a-96bc-456d-a747-4a0388eebf65 
  sid2 = 0eea70d7-9464-4bef-b62b-fc503de5cc60
  </code></pre>
  
  <p>A 2nd request prints the same <code>sid</code>, but a different <code>sid2</code></p>
  
  <pre><code>sid  = 87c7db3a-96bc-456d-a747-4a0388eebf65
  sid2 = 7355eeb0-f324-44a4-a61a-e629ba1a224f
  </code></pre>
  
  <p>Is the marker of <code>logRequest</code> evaluated once and reuse ?
  If this is the case, any hint on how to use string interpolation in the marker?</p>
  
  <p>Thanks</p>