anchor.diff:  If set to 'true', unlabeled documents are ignored and neither count as relevant or irrelevant. Set to 'false' (the default), they are treated as irrelevant.
   |=======================================================================
   
  +
   [float]
   ===== Mean reciprocal rank
   
  -For every query in the test suite, this metric calculates the reciprocal of the rank of the
  -first relevant document. For example finding the first relevant result
  -in position 3 means the reciprocal rank is 1/3. The reciprocal rank for each query
  -is averaged across all queries in the test suite to give the https://en.wikipedia.org/wiki/Mean_reciprocal_rank[mean reciprocal rank].
  +For every query in the test suite, this metric calculates the reciprocal of the 
  +rank of the first relevant document. For example finding the first relevant 
  +result in position 3 means the reciprocal rank is 1/3. The reciprocal rank for 
  +each query is averaged across all queries in the test suite to give the 
  +https://en.wikipedia.org/wiki/Mean_reciprocal_rank[mean reciprocal rank].
   
   [source,console]
   --------------------------------
  
positive.code: "relevant". Defaults to `1`.
  |=======================================================================
  
  [float]
  ===== Discounted cumulative gain (DCG)
  
  In contrast to the two metrics above, https://en.wikipedia.org/wiki/Discounted_cumulative_gain[discounted cumulative gain] takes both, the rank and the rating of the search results, into account.
  
  The assumption is that highly relevant documents are more useful for the user when appearing at the top of the result list. Therefore, the DCG formula reduces the contribution that high ratings for documents on lower search ranks have on the overall DCG metric.
  
  [source,console]
  --------------------------------
  
negative.code: <2> definition of the evaluation metric to calculate
  <3> a specific metric and its parameters
  
  The request section contains several search requests typical to your 
  application, along with the document ratings for each particular search request.
  
  [source,js]
  -----------------------------
  GET /my_index/_rank_eval
  {
      "requests": [
          {
              "id": "amsterdam_query", <1>
  