Owns the ETS tables that back real-time metering.

This process does nothing on the hot path — it merely creates and owns two public, named ETS tables so that writers (AuroraMeter.Counter) and readers hit ETS directly without a GenServer bottleneck:

• :aurora_meter_counters — {tenant_key, feature, period_start} => value
• :aurora_meter_dirty — the set of counter keys changed since the last flush

If this process crashes the tables are lost; its supervisor restarts it and the tables are recreated (counter state rehydrates lazily from the database).