Region selection for uptime checks

### Problem Statement

2-part

- multi-region checks. I can only see checks being performed from the US but our services are global (so this skews the response time statistics)
- The ability to choose which regions run checks, or filter by region so that I can see what the global and regional response is like

### Solution Brainstorm

_No response_

### Product Area

Stats

The issue does not name files, tests, or entry points. Start by locating the existing uptime-check statistics and check execution flows, then clarify the regional data model and selection or filtering behavior with maintainers. Done means multi-region checks and regional or global response statistics are defined and covered by tests.

Written by the indexing model from the issue text.