Expose user-defined meta-information via introspection API in form of directives

With growing popularity of IDL as a way to define a GraphQL schema, I think it would be quite beneficial to expose directive information via introspection API.

From what I can tell, the directive information is the only missing piece of information that is not obtainable via introspection API. For example in this schema definition:

```graphql
type User {
id: ID!
name: String
admin: Boolean! @important
}

type Query {
user: User
}
```

`@important` directive is only available at schema materialization time, but discarded as soon as schema is materialized from AST definitions.

One can see directives as a way to instruct server to construct the schema is specific way. But I would argue that directives have also a lot of potential as a way to expose additional meta-information about the schema. This may include things like: field cost/complexity (the use case I'm quite interested in), auth information, caching characteristics, and in general any kind of structured meta-information that is specific to a particular application or subset of applications. I think a lot of interesting use-cases and scenarios can emerge as soon as this meta-information is exposed via introspection. I can also imagine community working together on a set of common directives that may be adopted by client tooling and frameworks like apollo and relay. These common directives (exposed via introspection API) may provide deeper insights on the schema, relations between fields and objects, etc.

I described this feature in context of IDL, but in fact it's quite independent from it (though integrates with it very naturally). I was thinking about different ways how this kind of user-defined meta-information can be exposed via introspection API and I feel that directive-based approach is the most natural and well integrated way of doing it.

I would love to hear you opinions on this topic!