Fix zero-downtime reindexing docs count issue

What does this MR do?

While testing #214418 (closed) on staging I noticed that we count documents incorrectly with replicas > 0 because we used total documents count instead of primary documents count.

This MR fixes the incorrect documents count by using primaries indices stats instead of total

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
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• Style guides
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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