Cannot import Instance type

#### Environment details

  - `@google-cloud/bigtable` version: 2.3.2

#### Steps to reproduce

```
import { Bigtable, Instance } from "@google-cloud/bigtable";
```

gives an error, because `Instance` is not exported by the module.

I'd love to import `Instance` so that I can use it in type signatures (we're using Typescript).

