Concurrent/realtime stats

### Impacted plugin

Jetpack

### What

Add a realtime/concurrent visits/visitors reports for sites with large live audiences

### How

Similar to https://support.google.com/analytics/answer/1638637?hl=en, which can be helpful for sites with large live audiences, like news sites, that can use that metric to inform content management.

Considering it's a niche feature, it would be interesting as a paid add-on.

Start by reviewing the requested realtime/concurrent visits and visitors reports and the linked Google Analytics reference. Identify the Jetpack entry points and data sources involved, then clarify the paid add-on scope and acceptance criteria before implementation; done should mean the agreed report is available for sites with large live audiences.

Written by the indexing model from the issue text.