Premium Analytics: add metrics attribute to Traffic and Subscribers charts

[retrofox]

Fixes WOOA7S-1685

Proposed changes

• Declare a metrics array attribute (Edit: ArrayCheckboxField, relevance: 'high') on the two MetricTabsChart widgets, alongside their existing granularity attribute. example.attributes doubles as the defaults for new instances (all metrics enabled).
• Traffic chart: the tab definitions (views, visitors, likes, comments) move from use-traffic-chart.ts into widget.ts as the single source for the settings checkboxes and the chart tabs. The hook filters the tabs by the selected ids and skips a pair's request entirely (enabled: false) while neither of its fields is selected.
• Subscribers chart: the tab definitions (subscribers, paid) move from render.tsx into widget.ts; buildMetrics filters by the selected ids, and the Paid subscribers tab keeps rendering only when the site has paid subscribers.
• Both widgets show a "No metric selected" message when the selection is empty, and both stories expose a metrics check control plus the real attribute schema in the dashboard story's settings drawer.
• Drop the redundant description from both widget.ts files: widget.json is the source of truth for static widget metadata.

Related product discussion/links

• https://linear.app/a8c/issue/WOOA7S-1685/migrate-chart-widget-metric-tabs-to-metrics-array-attribute
• Follows WOOA7S-1682 (Store performance) and WOOA7S-1683 (Subscriber highlights).

Does this pull request change what data or activity we track or use?

No.

Testing instructions

1. Build Premium Analytics with dependencies: jp build packages/premium-analytics --deps
2. Open the Premium Analytics dashboard and add or edit a Traffic widget.
3. In the widget header, confirm the Metrics control appears inline as a compact menu with checkboxes for Views, Visitors, Likes, and Comments, next to the existing Group by control.
4. Toggle metrics and verify the chart tabs/dropdown only offer the selected metrics.
5. Uncheck Views and Visitors and confirm (network tab) that the views,visitors request is not sent; uncheck everything and confirm the "No metric selected" message.
6. Repeat with a Subscribers widget: checkboxes for Subscribers and Paid subscribers; on a site without paid subscribers the Paid tab stays hidden even while selected.
7. Add new instances of both widgets and confirm all metrics are selected by default.

[jp-launch-control]

Code Coverage Summary

This PR did not change code coverage!

That could be good or bad, depending on the situation. Everything covered before, and still is? Great! Nothing was covered before? Not so great. 🤷

Full summary · PHP report

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