title: terraform google_compute_instance - set variables instead of static labels
body: <p>i'm trying to set variable in the key section of the labels when
  creating <code>google_compute_instance</code></p>
  
  <p>this is possible:</p>
  
  <pre><code>labels {
      "billing-zone"          = "${var.zone}"
    }
  </code></pre>
  
  <p>but this:</p>
  
  <pre><code>labels {
      "${var.zone}"          = ""
    }
  </code></pre>
  
  <p>is not. ( the var can be only in the value section )
  is there anyway to pass that?</p>
  
  <p>Thank you.</p>