Expose new API for monitoring

Now we don't have any framework or interface to monitor the datastore. Here's one spec witch is not approved in https://blueprints.launchpad.net/trove/+spec/lightweight-monitoring-framework , but it's too complicated and doesn't integrate with Ceilometer.

We're proposing another way for monitoring by exposing the new API. The guestagent should provide a method to get database metrics and each datastore should implement this method. The trove-api is able to get metrics from this API and Ceilometer can get them as well.

This is the simplest way to support datastore monitoring without affecting the main framework. With Ceilometer, we don't need to store the metrics and setup the alert for trove users. It's simple but useful feature for Database as a Service.