Define a merged subscription with multiple subscription types.

*[See also the Book](https://async-graphql.github.io/async-graphql/en/merging_objects.html).*

# Macro attributes

| Attribute     | description               | Type     | Optional |
|---------------|---------------------------|----------|----------|
| name          | Object name               | string   | Y        |
| extends       | Add fields to an entity that's defined in another service | bool | Y |
| visible       | If `false`, it will not be displayed in introspection. *[See also the Book](https://async-graphql.github.io/async-graphql/en/visibility.html).* | bool | Y |
| visible       | Call the specified function. If the return value is `false`, it will not be displayed in introspection. | string | Y |

# Examples

```rust
use async_graphql::*;
use futures_util::stream::Stream;

#[derive(Default)]
struct Subscription1;

#[Subscription]
impl Subscription1 {
    async fn events1(&self) -> impl Stream<Item = i32> {
        futures_util::stream::iter(0..10)
    }
}

#[derive(Default)]
struct Subscription2;

#[Subscription]
impl Subscription2 {
    async fn events2(&self) -> impl Stream<Item = i32> {
        futures_util::stream::iter(10..20)
   }
}

#[derive(MergedSubscription, Default)]
struct Subscription(Subscription1, Subscription2);
```