observability(runway): add artifact-registry remote repositories dashboard

Adds dashboards/artifact-registry-gke/remote.dashboard.jsonnet, a detail dashboard for the kind=remote upstream-proxy read paths: the upstream requests each format proxy makes, the cache verdicts and fills behind them, the container registry's upstream auth handshake, and the proxy-side faults and retention writes. All from the gitlab_artifact_registry_{npm,maven,oci}_remote_* and gitlab_artifact_registry_remote_* metrics.

Like the other detail dashboards, it pins the Mimir - Runway datasource and replaces the stock environment template. $PROMETHEUS_DS defaults to the gprd datasource and the stock template assumes the GitLab.com gstg/gprd convention, while Runway labels app metrics environment=staging/production, so with the defaults every panel comes up empty.

Each format proxy names its own cache dimension (npm_cache_path, cache_path, artifact) and classifies the shared outcome label differently, so the upstream and cache rows carry one panel per format rather than one summed panel. The panel descriptions say so, because the label names invite the summed query.

The type:artifact-registry-gke tag is what lists the dashboard in the overview's detail dropdown.

Remote repositories have staging traffic only today, so panels with data in staging and none in production are expected.

Preview: https://dashboards.gitlab.net/d/jpereira-verify-ar-remote

Related to gitlab-org/ops/artifact-registry#354 (closed)