observability(runway): add AR query outcome and pool churn panels

Extends dashboards/artifact-registry-gke/db.dashboard.jsonnet with panels for the newly shipped AR database metrics: query error rate by name, query failure share, query cancellations by reason (client/deadline), and connection pool churn (opens and both destroy counters as separate series, deliberately unsummed).

Layout: both rows now wrap at three panels per line (layout.grid, cols=3, rowHeight=10), matching the runway postgres common graphs' convention for 3-across grids.

Also plumbs legend_hideEmpty through timeSeries/multiTimeSeries in libsonnet/grafana/time-series/panel.libsonnet (defaulted, proven inert by byte-identical recompiles of untouched dashboards) and disables it on the six healthy-zero panels, so their legend rows stay visible when the value is a flat zero — which is their normal state.

The new metric families exist in staging today and reach production on the next AR rollout; the panels read No data there until then.

Rendered copy for review: https://dashboards.gitlab.net/d/jpereira-verify-ar-db