Unable to compose schema for discriminator with index

Sorry if this is covered somewhere else, but I'm running into issues trying to compose a schema for a discriminator with an index. `graphql-compose-mongoose` wants to create sort types for my model, but since I'm using the default discriminator key of `__t` I'm getting the following error:

```
Name "__T_DESC" must not begin with "__", which is reserved by GraphQL introspection.
```

Has anyone figured out how to get around this? Is there some way to alias that field? We're not in a position to change our discriminator keys at the moment.

Here's a basic example:

```typescript
import mongoose from 'mongoose';
import { composeMongoose } from 'graphql-compose-mongoose';

test('discriminator index', async () => {
const animalSchema = new mongoose.Schema({
name: mongoose.Schema.Types.String,
age: mongoose.Schema.Types.String,
});

const dogSchema = new mongoose.Schema({
breed: mongoose.Schema.Types.String,
});
dogSchema.index({ __t: 1, breed: 1 });

const Animal = mongoose.model('Animal', animalSchema);
const Dog = Animal.discriminator('Dog', dogSchema);

await Dog.create([
{
name: 'spot',
age: 7,
breed: 'beagle',
},
{
name: 'dash',
age: 3,
breed: 'border collie',
},
{
name: 'india',
age: 5,
breed: 'newfoundland',
},
]);

// @ts-ignore
const DogTC = composeMongoose(Dog);

const composer = new SchemaComposer();
composer.add(DogTC);
composer.Query.addFields({
dogs: DogTC.mongooseResolvers.findMany(),
});

const result = await graphql.graphql({
schema: composer.buildSchema(),
source: `
query fetchDogs {
dogs {
_id
name
age
breed
}
}
`,
});

expect(result.data).toHaveLength(3);
});
```

This fails and the result object looks like:

```json
{
"errors": [
{
"message": "Name \"__T_ASC\" must not begin with \"__\", which is reserved by GraphQL introspection."
},
{
"message": "Name \"__T_DESC\" must not begin with \"__\", which is reserved by GraphQL introspection."
},
{
"message": "Name \"__T__BREED_ASC\" must not begin with \"__\", which is reserved by GraphQL introspection."
},
{
"message": "Name \"__T__BREED_DESC\" must not begin with \"__\", which is reserved by GraphQL introspection."
}
]
}
```