Validate if field groups don't have units and metric types defined

[mtojek]

What does this PR do?

This PR introduces another semantic validation check to verify if field groups don't use units and metric types.

Why is it important?

Field groups shouldn't use units and metric types.

Checklist

• I have added test packages to test/packages that prove my change is effective.
• I have added an entry in versions/N/changelog.yml.

Related issues

• No meta info for groups integrations#1091

[elasticmachine]

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Build stats

• Build Cause: Pull request #183 updated

• Start Time: 2021-06-08T10:10:22.310+0000

• Duration: 2 min 13 sec

• Commit: eab7a55

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[mtojek]

Ouch, forgot about metric_type. Let me fix the PR.

[fearful-symmetry]

not 100% familiar with the package-spec, but otherwise LGTM.