Add linked account information into billing metricset

[kaiyan-sheng]

What does this PR do?

This PR is to backport elastic/beats#26285 into AWS package.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• If I'm introducing a new feature, I have modified the Kibana version constraint in my package's manifest.yml file to point to the latest Elastic stack release (e.g. ^7.13.0).

[elasticmachine]

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Build stats

• Start Time: 2021-07-20T17:19:20.250+0000

• Duration: 14 min 57 sec

• Commit: bde1650

Test stats 🧪

Test	Results
Failed	0
Passed	247
Skipped	0
Total	247

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[aspacca]

+1