observability(runway): add artifact-registry HTTP API dashboard

Adds dashboards/artifact-registry-gke/http.dashboard.jsonnet, a detail dashboard for the HTTP API: traffic, errors, latency and payload sizes, broken down by route, method and status code, all from the gitlab_artifact_registry_http_* metrics.

The dashboard 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 env=staging/production, so with the defaults every panel comes up empty.

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

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

Part of gitlab-org/ops/artifact-registry#354 (closed).