GAX-JAVA: Allow users to customize ChannelPoolSettings RESIZE_INTERVAL and MAX_RESIZE_DELTA

[lkleeven]

We are creating our own in company Google Pub/Sub client. To improve performance we've recently switched to using the InstantiatingGrpcChannelProvider. This already gave a performance improvement, however to be able to deal with big spikes of load, we'd like to be able to also set the resize interval and max resize delta, currently present as package private static fields in ChannelPoolSettings. This would allow us to set clients up to respond faster to big spikes in load.

We've considered keeping a higher minimum channel count open, but that feels rather wasteful, especially if the spikes are few and far between.

We are willing to contribute if that helps to speed things up. And are also open to suggestions on how to otherwise deal with these load spikes from a client perspective.