Synthetic Monitoring in Sentry

### Problem Statement

A customer would like to have synthetic monitoring capabilities to Sentry to proactively detect issues before they impact real users.

Synthetic monitoring would allow teams to create automated test flows that simulate user interactions with an application. This would help identify performance issues, downtime, and regressions in real-time, even before users experience them.

### Solution Brainstorm

_No response_

### Product Area

Other

The issue provides only a high-level problem statement and names no files, tests, or implementation entry points. Start by clarifying the intended synthetic monitoring scope, user flows, and product area before locating relevant monitoring components. Done would require an agreed design and an implemented, tested capability that proactively detects issues.

Written by the indexing model from the issue text.