title: Grafana Elastic-Datasource not finding time field
body: <p>Given following mapping:</p>
  
  <pre><code>{
    "iot_data" : {
      "mappings" : {
        "device_data" : {
          "properties" : {
            "device_id" : {
              "type" : "keyword",
              "cql_collection" : "singleton",
              "cql_partition_key" : true,
              "cql_primary_key_order" : 1
            },
            "nano_since_epoch" : {
              "type" : "long",
              "cql_collection" : "singleton",
              "cql_primary_key_order" : 2
            },
            "sensor_id" : {
              "type" : "keyword",
              "cql_collection" : "singleton",
              "cql_primary_key_order" : 3
            },
            "source" : {
              "type" : "keyword",
              "cql_collection" : "singleton"
            },
            "time" : {
              "type" : "date",
              "cql_collection" : "singleton"
            },
            "unit" : {
              "type" : "keyword",
              "cql_collection" : "singleton"
            }
            "value" : {
              "type" : "double",
              "cql_collection" : "singleton"
            },
            "week_first_day" : {
              "type" : "date",
              "cql_collection" : "singleton",
              "cql_partition_key" : true,
              "cql_primary_key_order" : 0
            }
          }
        }        
      }
    }
  }
  </code></pre>
  
  <p>I´m trying to add a datasource to grafana, but it keeps on telling me that no date field has been found. What am I doing wrong? Is grafana unable to handle the "long" field as time field? 
  We have data that is inside of the same ms since epoch, that is why nano_seconds is used.</p>
  
  <p><a href="https://i.stack.imgur.com/CF4J7.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/CF4J7.png" alt="enter image description here"></a></p>
  
  <p>I am aware, that grafana needs an Date field as described here:
  <a href="https://github.com/grafana/grafana/issues/4321#issuecomment-282467390" rel="nofollow noreferrer">https://github.com/grafana/grafana/issues/4321#issuecomment-282467390</a></p>
  
  <p>Is it somehow possible to use a non-date field for x-axis with the elastic datasource?</p>