anchor.diff:  
   [source,js]
   --------------------------------------------------
  -curl -XPOST 'http://localhost:9200/_field_stats?level=indices' -d '{
  +curl -XPOST "http://localhost:9200/_field_stats?level=indices" -d '{
      "fields" : ["rating"]
   }'
   --------------------------------------------------
  
positive.code: --------------------------------------------------
  
  <1> The `_all` key indicates that it contains the field stats of all indices in the cluster.
  ==================================================
  
  .Indices level field statistics
  ==================================================
  
  [source,js]
  --------------------------------------------------
  GET /_field_stats?fields=rating,answer_count,creation_date,display_name&level=indices
  --------------------------------------------------
  
  [source,js]
  --------------------------------------------------
  
negative.code: 
  [source,js]
  --------------------------------------------------
  curl -XPOST "http://localhost:9200/_field_stats?level=indices" -d '{
     "fields" : ["answer_count"] <1>
     "index_constraints" : { <2>
        "creation_date" : { <3>
  