Frontend: Add totalEventCount to User metrics table

What does this MR do and why?

Adds the totalEventCount metric to the user metrics table in the analytics dashboard. This metric, now available from the backend, provides per-user aggregated event counts to help teams understand individual user engagement with AI features.

Key changes:

• Extends the data source to fetch totalEventCount from the GraphQL API
• Updates the user metrics table visualization to display the new metric
• Enables tracking of total AI feature usage per user across the organization

This completes the user metrics table implementation by incorporating the total event count data that was previously unavailable.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. You will need an GitLab Ultimate license
2. Setup Clickhouse in your GDK: https://docs.gitlab.com/development/database/clickhouse/clickhouse_within_gitlab/#gdk-setup
3. Run FILTER=ai_usage_stats bundle exec rake db:seed_fu
4. Navigate to Analyze > Analytics dashboards > GitLab Duo and SDLC trends for any group or project
5. Validate that the GitLab Duo usage by user table is rendered

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #574426