title: Provide deeper swarm nodes-related metrics
body: <p>I would like to be able to be able to monitor more accurately the manager nodes within a swarm cluster.
  I expect (eg) such a metrics : </p>
  
  <pre><code>node_swarm{instance="10.0.0.14:9100",job="node",node_id="123",node_name="node_7",node_availabilty="drain",node_manager_status="Reachable",node_status="unreachable"} 1  
  node_swarm{instance="10.0.0.145:9100",job="node",node_id="456",node_name="node_8",node_availabilty="Active",node_manager_status="Unreachable",node_status="down"} 1
  </code></pre>
  
  <p>... if it could also provide the labels of the nodes, these would be the icing on the cake!
  ...</p>
  
  <p>But for now, I didn't manage to provide such with node_exporter (or cAdvisor)... I may have missed some clue, but they don't sound like they provided such hints.</p>
  
  <p>Do you have any idea on how I could provided such metrics? </p>
  
  <p>I am quite sure I could code them from scratch (or submit a feature request), but I don't want to reinvent the Wheel if it already exists!</p>