Frontend: Add GitLab Duo agent/flow users

Proposal

Add a new DAP centric stat tile showing GitLab Duo agent/flow users on the analytics dashboard. This panel will replace the "GitLab Duo Code Suggestions acceptance rate" panel.

🖼️ Design

DAP_overview_metrics_-_absolute_values

⚠️ UI text on mock maybe out-of-date, please use this spreadsheet to the SSoT.

Edited by 🤖 GitLab Bot 🤖