Provide metrics for ZooKeeper clients

[nicktelford]

Ideally, ZooKeeper client instances should be instrumentable with metrics; specifically, each request type (e.g. "create", "set", "getACL", etc.) should be timed to measure request latency etc.

This is problematic because:

• ZooKeeper is a class, there is no interface that would allow us to naturally extend the API.
• The constructor of ZooKeeper initialises the thread-pool and socket connections for the client, which prevents us from writing tests for any sub-classes.