Metric alerts - Support equations/formulas in Metric Alerts to calculate derived metrics like failure rate

### Problem Statement

Currently, Metric Alerts only support single metric queries. This makes it impossible to calculate percentage-based metrics like:
- Failure rate: failed_requests / total_requests
- Error rate: errors / (errors + successes)
- Success rate: successful / total

Current Workaround Gaps
Discover: Has equations but can't query spans or custom metrics
Explore: Can query spans but no equations
Metric Alerts: Works with spans/custom metrics but only 1 metric (no equations)

Use Case
Alert on API failure rate based on HTTP status codes from mobile apps (Android/iOS). Need to avoid false positives from varying traffic volumes (day/night, week-over-week).

### Solution Brainstorm

_No response_

### Product Area

Alerts

Start with the Metric Alerts area and compare its current single-metric behavior with the equations support described for Discover and the span-query support described for Explore. Define the scope for querying multiple metrics and calculating derived rates, then verify that API failure-rate alerts work across changing traffic volumes.

Written by the indexing model from the issue text.