feat: full attribute filters on metrics widgets, create_dashboard where/unit/series count - #1388

🟢 Confidence 9/10 · safe to merge
Both follow-up fixes (quoteWhereValue, numeric present guard) are correct, pinned by tests, and reach no other caller.
_{quality 100/100 · no findings · tests covered · risk medium}

Adds full attr.* operator support to metric datapoint filters end to end, plus where/unit on create_dashboard simple widgets and a seriesCount in widget validation. The follow-up fixes at this head are correct; safe to merge.

• buildAttrFilterCondition requires a non-empty label for attr.x < / <= / > />=
• create_dashboard service_name goes through quoteWhereValue in the widget where clause
• Metrics queries and the runtime take the whole attributeFilters list
• WidgetInspectionEntry reports seriesCount from the largest query

_{b4ebf3c · Updated on every push. Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}