Frontend: Add RCA usage to User metrics table

Proposal

The user metrics table is being introduced in Frontend: User metrics table - first iteration (#574420 - closed) • Ezekiel Kigbo • 18.7 with data available at the time of creating this issue.

A per user RCA (troubleshoot job) event count metric is being exposed via https://gitlab.com/gitlab-org/gitlab/-/issues/570991+s.

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

Implementation

• Extend data source
• Update table visualization
• Update documentation