Alerts for queue monitoring metrics

### Problem Statement

Sentry's queue monitoring provides valuable metrics like average time in queue and average processing time, but currently doesn't support alerting based on these metrics. This creates a reactive monitoring approach where performance degradation goes unnoticed until manual dashboard checks or user complaints occur, leading to delayed incident response and potential user impact.

### Solution Brainstorm

Add alert rule capabilities for queue monitoring metrics, allowing users to:

- Set thresholds for average queue time and processing time
- Support both absolute thresholds and percentage-based changes over time periods

This would enable proactive monitoring and faster incident detection for queue-related performance issues.

### Product Area

Alerts

Start by reviewing Sentry's queue monitoring metrics and the Alerts product area. Define how alert rules should use average queue time and average processing time, including absolute thresholds and percentage changes over time periods. The work is done when users can configure these rules and receive alerts for queue-related degradation.

Written by the indexing model from the issue text.