Introduce internal/metrics exposing the four metric families spec §23
mandates at minimum: felis_servers_total (gauge by desired state),
felis_start_duration_seconds (histogram with Minecraft cold-start
buckets), felis_image_build_failures_total and
felis_reaper_worlds_deleted_total (counters). Collectors are
package-level vars so any subsystem records without an import cycle;
Register wires them into a prometheus.Registerer and is idempotent.
Wire registration into the operator against controller-runtime's global
Registry, so /metrics on the manager's existing metrics endpoint carries
the felis_* families. Instrument the reaper to increment
felis_reaper_worlds_deleted_total in lockstep with Summary.WorldsReaped,
at the one point a world's PVC has actually been deleted.
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