Stop emitting advice when enum has unknown value

[lmolkova]

Enums are expected to have unknown values, if some of them need strict validation, it can come as custom policy but it can't be default applied to all enums.

E.g. error.type is one of the most common semconv attributes and it's designed to be an open set.

Adding weaver live check to a typical app with HTTP or DB calls brings A LOT of these

PS: I love the ability to emit logs to my backend!

[jerbly]

I think this should be solved as part of #1248

The information and improvement level findings are useful. This one regarding additional enum variants is very useful to catch a variant that ought to be part of the spec but has been missed.