Frontend: Add totalEventCount to User metrics table

Proposal

The user metrics table is being introduced in Frontend: User metrics table - first iteration (#574420) • Ezekiel Kigbo • 18.6 with data available at the time of creating this issue.

A per user total event count metric is being exposed via Backend: Total event counts per user metrics (#574400) • Amr Taha • 18.6.

We need to add the totalEventCount metric to the user metrics table as soon as it becomes available.

Implementation

• Extend data source
• Update table visualization
• Update documentation