Provide metrics for Curator instances

[nicktelford]

We should optionally instrument Curator instances with metrics, specifically for requests (e.g. "create", "get", etc.)

This would be independent to any metrics on its underlying ZooKeeper instances as it would include client-side behaviour, such as retrying failed requests, or creating new ZooKeeper connections when the current client disconnects.

The Curator API is very well designed, so this shouldn't be a substantial problem.