[Transform] Deprecation warnings do not get forwarded in preview

[hendrikmuhs]

The stack upgrade to 8.x lists deprecation's for painless: https://www.elastic.co/guide/en/elasticsearch/reference/8.0/migrating-8.0.html#breaking_80_painless_changes

When a user uses deprecated syntax the user should get warnings when he uses deprecated syntax. This works when issuing a query but not for _preview and PUT, repro steps below.

The issue mainly targets 7.17, however the fix should go into the main branch, too. There might be other deprecation's in the future. (however the latest 8.x, e.g. 8.1, is sufficient)

Note: The same applies for query runtime fields that can be defined as part of source.

Reproduction on a 7.17 build:

POST _transform/_preview
{
  "source": {
    "index": "kibana_sample_data_logs"
  },
  "pivot": {
    "group_by": {
      "@timestamp": {
        "date_histogram": {
          "field": "@timestamp",
          "calendar_interval": "1m"
        }
      }
    },
    "aggregations": {
      "bytes.avg": {
        "avg": {
          "field": "bytes"
        }
      },
      "millis": {
        "scripted_metric": {
          "init_script": "state.m = 0",
          "map_script": " state.m = doc['@timestamp'].value.millis;",
          "combine_script": "return state.m;",
          "reduce_script": """
          def last = 0; for (s in states) {last = s;} return last;      
          """
        }
      }
    }
  }
}

A query returns a warning:

GET kibana_sample_data_logs/_search
{
 "aggregations": {
      "bytes.avg": {
        "avg": {
          "field": "bytes"
        }
      },
      "millis": {
        "scripted_metric": {
          "init_script": "state.m = 0",
          "map_script": " state.m = doc['@timestamp'].value.millis;",
          "combine_script": "return state.m;",
          "reduce_script": """
          def last = 0; for (s in states) {last = s;} return last;      
          """
        }
      }
    } 
}

[elasticmachine]

Pinging @elastic/ml-core (Team:ML)