model: change Host.IP to an array

[axw]

Motivation/summary

ECS states that host.ip should hold an array of values. Let's align with ECS before we settle on a stable encoding.

Checklist

• Update CHANGELOG.asciidoc
  - [ ] Update package changelog.yml (only if changes to apmpackage have been made)
• Documentation has been updated

How to test these changes

1. Run apm-server, send events
2. Check a transaction sample's metadata, host.ip should be rendered sensibly in the flyout in the UI
3. Check the document's _source, host.ip should be an array (even if there's just one IP).

Related issues

#4120

[apmmachine]

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Build stats

• Start Time: 2021-11-23T08:09:16.084+0000

• Duration: 40 min 17 sec

• Commit: dfaed22

Test stats 🧪

Test	Results
Failed	0
Passed	5622
Skipped	19
Total	5641

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[marclop]

Verified with 9bb5e4c:

$ curl -H "Content-type: application/x-ndjson" --data-binary @testdata/intake-v2/transactions.ndjson http://localhost:8200/intake/v2/events

GET traces-apm*/_search
{
  "_source": [false], 
  "fields": ["host.ip"],
  "query": {
    "exists": {
      "field": "host.ip"
    }
  }
}

{
  "took" : 840,
  "timed_out" : false,
  "_shards" : {
    "total" : 2,
    "successful" : 2,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 5,
      "relation" : "eq"
    },
    "max_score" : 1.0,
    "hits" : [
      {
        "_index" : ".ds-traces-apm-default-2021.12.21-000001",
        "_id" : "gDPr230ByDpadNqDk0pg",
        "_score" : 1.0,
        "_source" : { },
        "fields" : {
          "host.ip" : [
            "127.0.0.1"
          ]
        }
      },
      {
        "_index" : ".ds-traces-apm-default-2021.12.21-000001",
        "_id" : "gTPr230ByDpadNqDk0pg",
        "_score" : 1.0,
        "_source" : { },
        "fields" : {
          "host.ip" : [
            "127.0.0.1"
          ]
        }
      },
      {
        "_index" : ".ds-traces-apm-default-2021.12.21-000001",
        "_id" : "gjPr230ByDpadNqDk0pg",
        "_score" : 1.0,
        "_source" : { },
        "fields" : {
          "host.ip" : [
            "127.0.0.1"
          ]
        }
      },
      {
        "_index" : ".ds-traces-apm-default-2021.12.21-000001",
        "_id" : "gzPr230ByDpadNqDk0pg",
        "_score" : 1.0,
        "_source" : { },
        "fields" : {
          "host.ip" : [
            "127.0.0.1"
          ]
        }
      },
      {
        "_index" : ".ds-traces-apm-default-2021.12.21-000001",
        "_id" : "hDPr230ByDpadNqDk0pg",
        "_score" : 1.0,
        "_source" : { },
        "fields" : {
          "host.ip" : [
            "127.0.0.1"
          ]
        }
      }
    ]
  }
}

It is also displayed correctly in the APM UI: