Add a polymorphic owner to `Metric` so it can belong to either a service (case of existing non-APIAP services and also APIAP products in APIAP v2) or to a Backend API.

Dev note

No need to "migrate" existing metrics to the services' backend APIs, but only fill the `owner_id` and `owner_type` (via db migration) and make sure a backend API can own a metric directly. This will easy the support of APIAP and non-APIAP services in parallel for a moment and exempt us from having to run a heavy migration that would potentially require resync with backend/apisonator.