Add metadata to API Keys created by server

[axw]

Motivation/summary

Add application: "apm" in the metadata of API Keys created by APM Server, so they can later be identified as having been created by/for APM. The metadata must not be used for auth; it is purely for informational purposes.

Metadata will be shown when running apm-server apikey info with the --json flag, but not in the default text format.

Checklist

• Update CHANGELOG.asciidoc

How to test these changes

1. Create an API Key with apm-server apikey create
2. Run apm-server apikey info --json --id <id-from-step-1>, check the metadata shows {"application": "apm"}

Related issues

#3461
elastic/kibana#77966

[apmmachine]

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Build stats

• Build Cause: Pull request #5090 opened

• Start Time: 2021-04-14T03:36:17.806+0000

• Duration: 42 min 40 sec

• Commit: c767d0f

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Failed	0
Passed	6274
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Total	6394

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[axw]

@sqren tagged you for review just to check that the metadata I've proposed here is OK.

[stuartnelson3]

Tested against commit https://github.com/elastic/apm-server/commits/0aa1b50a5b680be19556d53f870fa9723bdc9a03, {"application": "apm"} is present.

# ./apm-server version
apm-server version 7.13.0 (amd64), libbeat 7.13.0 [0aa1b50a5b680be19556d53f870fa9723bdc9a03 built 2021-04-27 15:36:50 +0000 UTC]
# ./apm-server apikey create -E output.elasticsearch.username=admin -E output.elasticsearch.password=changeme
API Key created:

Name ........... apm-key
Expiration ..... never
Id ............. KsKLMXkBBOTslOUzyQld
API Key ........ tdeCQFN2S-akzUIouGQEJw (won't be shown again)
Credentials .... S3NLTE1Ya0JCT1RzbE9VenlRbGQ6dGRlQ1FGTjJTLWFrelVJb3VHUUVKdw== (use it as "Authorization: APIKey
<credentials>" header to communicate with APM Server, won't be shown again)
~/w/a/apm-server-7.13.0-linux-x86_64 (master %=)
# ./apm-server apikey info --json --id KsKLMXkBBOTslOUzyQld -E output.elasticsearch.username=admin -E output.ela
sticsearch.password=changeme
{
        "api_keys": [
                {
                        "id": "KsKLMXkBBOTslOUzyQld",
                        "name": "apm-key",
                        "creation": 1620033915229,
                        "invalidated": false,
                        "username": "admin",
                        "metadata": {
                                "application": "apm"
                        }
                }
        ]
}