ETS owner for the named ETS table — one row per registered Instrument keyed by downcased_name (spec metrics/api.md §Instrument identity L190-L191). InstrumentationScope is hardcoded SDK-wide (project memory project_minikube_hardcode_decisions § Follow-on #457), so it is omitted from the key.

A GenServer that owns the table so its lifetime matches the SDK supervisor and dies with it. The table is public with read_concurrency / write_concurrency so any process reads/writes without going through this server — the GenServer is not on the hot path. Same pattern as Otel.Trace.SpanStorage.