Don't send runtime field mappings when the field is not in use

[ghudgins]

Describe the feature
Don't send runtime mapping when they aren't in use

Describe a specific use case for the feature:
When I have a runtime field defined
And I am not using it in a visualization
I need Lens to not send the runtime mapping
So I can avoid any extra cost having this mapping has <-- assumption here...does it cost anything? if not we can close this issue

See dont-request-me in below vis (that is only doing a date histogram of document counts)

{
  "aggs": {
    "0": {
      "date_histogram": {
        "field": "@timestamp",
        "fixed_interval": "30s",
        "time_zone": "America/New_York",
        "extended_bounds": {
          "min": 1660752918150,
          "max": 1660753818150
        }
      }
    }
  },
  "size": 0,
  "fields": [
    {
      "field": "@timestamp",
      "format": "date_time"
    },
    {
      "field": "timestamp",
      "format": "date_time"
    },
    {
      "field": "utc_time",
      "format": "date_time"
    }
  ],
  "script_fields": {},
  "stored_fields": [
    "*"
  ],
  "runtime_mappings": {
    "hour_of_day": {
      "type": "long",
      "script": {
        "source": "emit(doc['timestamp'].value.getHour());"
      }
    },
    "dont-request-me": {
      "type": "keyword",
      "script": {
        "source": "emit('yo');"
      }
    }
  },
  "_source": {
    "excludes": []
  },
  "query": {
    "bool": {
      "must": [],
      "filter": [
        {
          "range": {
            "@timestamp": {
              "format": "strict_date_optional_time",
              "gte": "2022-08-17T16:15:18.150Z",
              "lte": "2022-08-17T16:30:18.150Z"
            }
          }
        }
      ],
      "should": [],
      "must_not": []
    }
  }
}

[elasticmachine]

Pinging @elastic/kibana-vis-editors @elastic/kibana-vis-editors-external (Team:VisEditors)

[ghudgins]

CC @elastic/kibana-app-services in case this is actually an issue in your domain

[flash1293]

Elasticsearch will only evaluate the fields if required (e.g. they are used in a filter or in an aggregation).

There is a cost attached to them as they have to be sent over the wire with every request so if there are a lot of large runtime fields it might have a certain impact because uploading bytes and Json serialization and parsing isn’t free.

however I don’t think it’s worth pursuing this as I don’t think it would make a significant difference in practice and it’s very hard to tell whether a runtime field is used or not (the benefit of runtime fields is that they behave like regular fields in almost every case so you can use them almost everywhere, in query DSL and so on).

I’d say let’s wait whether customers actually run into issues with this

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[kertal]

Closing this because it's not planned to be resolved in the foreseeable future. It will be tracked in our Icebox and will be re-opened if our priorities change. Feel free to re-open if you think it should be melted sooner.