Validate maximum number of metadata that can be set on an entity

[viniarck]

Currently, there's no upperbound when setting a metadata attribute of a GenericEntity (LInk, Switch, Interface), this can lead to massive objects stored and even vulnerability for unauthenticated endpoints. Since this data will also be written on Mongo it's desirable to validate both how large this object is and how many keys.

Maybe we can limit up to 200 metadata keys for now, and also measure on average the expected size of that object, let me know if that sounds reasonable for the number of metadata keys we'll have in prod, what do you think?