Proposal: preserve parameterized types

[snytkine]

Would it be possible to preserve parameterized metadata?

For a code like this one

class UserService {

@service 
getUser(id:number):Promise<User>

}

It's currently possible to get the value of design:returntype using reflect-metadata but the value is limited to Promise prototype

It would be very helpful to also somehow preserve the type of Promise as well. This would be very helpful for writing a library that generates documentation from code, possibly a swagger file from inspecting signatures of controllers or generates schema from a class.

[DanielRosenwasser]

Closing as a duplicate of #3015.