Migrate root_cause_analysis_users_count metric to new data source

Pavel Shutsinrequested to merge
561471-update-troubleshootusageservice-to-use-new-ch-data-source into master

What does this MR do and why?

Migrate root_cause_analysis_users_count metric to new data source. Underlying data was migrated in Add data migration to migrate ai usage tables i... (#549795 - closed)

No user behavior changes.

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Related to #561471 (closed)

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