fix: refactor the showcase stream method collect

[summer-ji-eng]

The compiler complains the augment type error for showcase test on stream method collect

test/fixtures/google-gax-packaging-test-app/src/index.ts:245:39 - error TS2769: No overload matches this call.
  Overload 1 of 2, '(options?: CallOptions | undefined, callback?: Callback<IEchoResponse, IEchoRequest | null | undefined, {} | null | undefined> | undefined): CancellableStream', gave the following error.
    Type '(err: Error, result: { content: string; }) => void' has no properties in common with type 'CallOptions'.
  Overload 2 of 2, '(callback?: Callback<IEchoResponse, IEchoRequest | null | undefined, {} | null | undefined> | undefined): CancellableStream', gave the following error.
    Argument of type '(err: Error, result: {    content: string;}) => void' is not assignable to parameter of type 'Callback<IEchoResponse, IEchoRequest | null | undefined, {} | null | undefined>'.
      Types of parameters 'err' and 'err' are incompatible.
        Type 'Error | null | undefined' is not assignable to type 'Error'.
          Type 'undefined' is not assignable to type 'Error'.

245   const stream = await client.collect((err: Error, result: {content: string}) => {