[Stack Monitoring] Switch cgroup memory fields to keyword

[matschaffer]

What does this PR do?

Changes cgroup memory fields from long to string.

Per https://www.elastic.co/guide/en/elasticsearch/reference/current/cluster-nodes-stats.html they should be strings. data.go appears to also consider them strings, so just updating the field definitions might be enough.

Why is it important?

With the current mapping as long, ingest of elasticsearch node data can fail when elasticsearch responds with a string like 'max'.

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 (used existing suite)
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

So far I can't figure out how to get ES to respond with max in the cgroup memory setting:

$ curl -s -k -u elastic:'(password)' https://localhost:9200/_nodes/stats/os | jq '.nodes | values | .[].os.cgroup.memory'
{
  "control_group": "/",
  "limit_in_bytes": "9223372036854771712",
  "usage_in_bytes": "17973219328"
}

I'll need that before I can advise on how to test this.

Related issues

• Fixes [Stack Monitoring] Mapping for elasticsearch.node.stats.os.cgroup.memory.limit.bytes is incorrect #31765

Use cases

Stack Monitoring

Logs

Without this change you can get this error on ingest:

failed to parse field [elasticsearch.node.stats.os.cgroup.memory.limit.bytes] of type [long] in document with id '5NvEroEBXQqSgbZj8wR7'. Preview of field's value: 'max'\",\"caused_by\":{\"type\":\"illegal_argument_exception\",\"reason\":\"For input string: \\\"max\\\"\"}}, dropping event!","service.name":"metricbeat","ecs.version":"1.6.0"}

(from forum)

[elasticmachine]

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• Start Time: 2022-07-11T07:46:54.075+0000

• Duration: 55 min 27 sec

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Passed	3570
Skipped	884
Total	4454

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