Cherry-pick #16000 to 7.x: Make field names in Functionbeat ECS compliant

[kvch]

Cherry-pick of PR #16000 to 7.x branch. Original message:

What does this PR do?

This PR fixes an issue in the storage function of Functionbeat. ES was not able to index events coming from the function as one of its fields had a different type:

{
    "textPayload": "2020-01-31T15:03:10.758Z\tWARN\telasticsearch/client.go:516\tCannot index event publisher.Event{Content:beat.Event{Timestamp:time.Time{wall:0xf60c480, ext:63716079787, loc:(*time.Location)(nil)}, Meta:null, Fields:{\"agent\":{\"ephemeral_id\":\"d86f3440-cb23-431a-81b7-d3360017ad2b\",\"hostname\":\"localhost\",\"id\":\"1a0a7af8-b927-460f-9746-7f8eae8dd417\",\"type\":\"functionbeat\",\"version\":\"7.6.0\"},\"bucket\":\"test-elastic-bucket\",\"created\":\"2020-01-31T15:03:06.783Z\",\"ecs\":{\"version\":\"1.4.0\"},\"file\":\"Untitled\",\"host\":{\"name\":\"localhost\"},\"id\":\"951771144251331\",\"metageneration\":\"1\",\"read_timestamp\":\"2020-01-31T15:03:10.209Z\",\"resource\":{\"event_type\":\"storage#object\",\"name\":\"projects/_/buckets/test-elastic-bucket/objects/Untitled\",\"service\":\"storage.googleapis.com\",\"state\":\"\"},\"updated\":\"2020-01-31T15:03:06.783Z\"}, Private:interface {}(nil), TimeSeries:false}, Flags:0x1, Cache:publisher.EventCache{m:common.MapStr(nil)}} (status=400): {\"type\":\"mapper_parsing_exception\",\"reason\":\"object mapping for [file] tried to parse field [file] as object, but found a concrete value\"}",
    "insertId": "000000-4f927011-4af5-4876-b471-62889ff0796b",
    "resource": {
      "type": "cloud_function",
      "labels": {
        "project_id": "elastic-metricbeat",
        "region": "europe-west2",
        "function_name": "storage"
      }
    },
    "timestamp": "2020-01-31T15:03:10.758Z",
    "labels": {
      "execution_id": "951771144251331"
    },
    "logName": "projects/elastic-metricbeat/logs/cloudfunctions.googleapis.com%2Fcloud-functions",
    "trace": "projects/elastic-metricbeat/traces/571a3f3523594e819691f0672c32c413",
    "receiveTimestamp": "2020-01-31T15:03:12.107405704Z"
  }

Why is it important?

This problem prevented ES from indexing new storage events.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works