feat: Add metric widget to dashboard

[PranshuSrivastava]

Add metric widget to dashboard to a specific metric such as total cost, total o/p tokens etc.
Fixes: #1499
Preview of the widget:

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Note

Medium Risk
Introduces a new chart type and changes validation/schema to allow config.x to be optional, which could affect existing chart save/load paths if any code still assumes x is always present.

Overview
Adds a new ChartType.Metric that renders a single numeric KPI by summing the selected y column via a new MetricChart component, with empty-state handling.

Updates chart configuration/validation and export/persistence to make x optional for metric charts (UI validation, export dialog checks, and backend zod schema). Updates the dashboard editor to support metric charts by generating configs without dimensions, hiding dimension/limit/total controls, and emitting query structures with no dimensions while still applying the time filter behavior used for non-timeseries charts.

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