feat(tracemetrics): Add equation support in old tracemetric alerts

[narsaynorath]

Only adds the equation builder to old tracemetrics alerts

Best viewed with whitespace changes off since some component structures were indented to allow for the feature flag branch

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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Inconsistent projectIds fallback: undefined vs empty array

Medium Severity

The refactoring changed the projectIds fallback in the detector form path from [] (empty array, the prior behavior inside MetricToolbar) to undefined. The new legacy alert path in metricsEquationVisualizeField.tsx correctly uses [] as the fallback, matching the original behavior. This inconsistency means the MetricSelector in the detector form receives undefined instead of [] when no project is selected, which can change query behavior (e.g., fetching metrics for all projects rather than none).

Additional Locations (1)

• static/app/views/alerts/rules/metric/traceMetrics/metricsEquationVisualizeField.tsx#L27-L28

 

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[narsaynorath]

This is fine for now, I can follow up in another PR to tighten up what the proper filter should be in this case. Since project selection is important for setting alerts I don't expect this to be common, only on init maybe but even then, I should just disable the request if we don't have a project where we expect one.