[entityframeworkcore] Define stability requirements for ORM instrumentation

[alanwest]

Entity Framework is an ORM. As such, the instrumentation for it supports many database systems. The semantic conventions differ from system to system.

We must determine what unique requirements may exist in order to declare Entity Framework instrumentation stable.

This may not be an exhaustive list, but here's some things that differ depending on database system:

1. See footnote for db.namespace: "Semantic conventions for individual database systems SHOULD document what db.namespace means in the context of that system."
2. See footnote for network.peer.*: "Semantic conventions for individual database systems SHOULD document whether network.peer.* attributes are applicable."