Environment details

• OS: macOS 10.14
• Node.js version: v12.12.0
• npm version: 6.11.3
• @google-cloud/bigtable version: 2.2.1

Steps to reproduce

I have code that calls getInstances without defining a callback:

const [instances] = await bigtable.getInstances();

getInstances behaves according to its docstring:

If the callback is omitted, we'll return a Promise.

...but this behavior is not reflected in its generated type, which has return type void and no overloads:

getInstances(gaxOptions?: any, callback?: any): void;

This mismatch causes eslint to complain because void is a non-Promise.

Proposal

Declare an overload type:

getInstances(gaxOptions?: any, callback: any): void;
getInstances(gaxOptions?: any): Promise<Instance[]>;

Thanks!