Add custom fields to source of the JSON in metricbeat

Question

I know that we can add custom fields in metricbeat.yml file as,

fields:
  customfield1: value1
  customfield2: value2

Then the JSON output would be generated as,

{
  "@timestamp": "2018-03-15T17:00:04.392Z",
  "@metadata": {
    "beat": "metricbeat",
    "type": "doc",
    "version": "6.2.2"
  },
  "metricset": {
    "rtt": 801,
    "name": "status",
    "module": "kibana",
    "host": "localhost:5601"
  },
  "error": {
    "message": "error making http request: Get http://localhost:5601/api/status: dial tcp [::1]:5601: getsockopt: connection refused"
  },
  "fields": {
    "customfield1": "value1",
    "customfield2": "value2"
  },
  "beat": {
    "name": "TheNameOfBeat",
    "hostname": "localhost",
    "version": "6.2.2"
  }
}

But I want to add the custom fields to the source of the JSON message and not inside the "fields" object. Is this possible?

Something like,

{
      "@timestamp": "2018-03-15T17:00:04.392Z",
      "customfield1": "value1",
      "customfield2": "value2",
      "@metadata": {
        "beat": "metricbeat",
        "type": "doc",
        "version": "6.2.2"
      },
      "metricset": {
        "rtt": 801,
        "name": "status",
        "module": "kibana",
        "host": "localhost:5601"
      },
      "error": {
        "message": "error making http request: Get http://localhost:5601/api/status: dial tcp [::1]:5601: getsockopt: connection refused"
      },
      "beat": {
        "name": "TheNameOfBeat",
        "hostname": "localhost",
        "version": "6.2.2"
      }
    }

Answer 1

Found the solution in How can I parse custom Metricbeat dictionary for Kibana? Need to set the fields_under_root option to true.