[8.4](backport #1951) Fix fleet.migration.total log key overlap

[mergify]

This is an automatic backport of pull request #1951 done by Mergify.
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On branch mergify/bp/8.4/pr-1951
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	both modified:   CHANGELOG.next.asciidoc
	both modified:   internal/pkg/dl/migration.go

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[elasticmachine]

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• Start Time: 2022-10-06T17:32:19.607+0000

• Duration: 3 min 1 sec

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[mergify]

This pull request has not been merged yet. Could you please review and merge it @joshdover? 🙏