Allow customized options

[mingwho]

With this change, whoever using the library can pass their own subscriber options.
For example we can pass some of the following options

const subscriberOptions = {
  ackDeadline: 10,
  batching: {
    // https://googleapis.github.io/gax-nodejs/CallSettings.html}
    callOptions: {...},
    maxMessages: 3000,
    maxMilliseconds: 100
  },
  flowControl: {
    allowExcessMessages: true,
    maxBytes: os.freemem() * 0.2,
    maxExtension: Infinity,
    maxMessages: 100
  },
  streamingOptions: {
    highWaterMark: 0,
    maxStreams: 5,
    timeout: 300000
  }
};
subscribe(topic, subscription, onMessage, subscriberOptions)

If subscriberOptions is not provided, we will use default options. If subscriberOptions is {} or [] or "" or null or undefined, or anything that the Pubsub API can't understand, it will use default options.

Documentation is here: https://cloud.google.com/pubsub/docs/pull
Related comments on pubsub subscriber options here: googleapis/nodejs-pubsub#696

[joejean]

An empty object evaluates to true in JS. So, if options is an empty object {}, we will end up passing it to the subscription function instead of the defaultOptions, is that what we want?

[mingwho]

We can discuss what to do with this type of input. If the options is an empty object {}, the Pubsub library will use it's own default options, instead of the defaultOptions in this wrapper. This is not expected behavior. Maybe we can have a check to see (if subscriberOptions.ackDeadline || subscriberOptions.batching || subscriberOptions.flowControl || subscriberOptions.streamingOptions), then we use the subscriberOptions. Otherwise, use defaultOptions. What do you think?

[joejean]

If the Pubsub library will use it's default options when the passed options is an empty {}, then I think that's okay.

[mingwho]

Ok perfect 👍