Add dimensions apis for both dimension names and values

Currently, in order to get the list of unique dimension names or values for a given metric-name, you either need to call metric-list or metric-list with the --name argument. In a project with large amounts of data, the metric-list call is expensive -- especially if you aren't interested in the full result. metric-list is what's currently called from our grafana integration when templating dashboards.

This blueprint introduces two new APIs:

GET v2.0/metrics/dimensions/names
GET v2.0/metrics/dimensions/names/values

The dimensions/names/values interface will accept dimension name, and optional metric name, and return the unique values for that combination. This will be used by the grafana integration -- but without a metric name.

The dimensions/names will accept a metric name, and return all unique dimension names for that metric name. This isn't immediately needed for the grafana integration, but will likely be more performant than metric-list with the --name parameter (used currently).

The plan for implementation of this blueprint will likely occur in two patch sets -- one for each of the new APIs. Both patch sets must work with vertica and influxdb, as well as in the java and python versions of the API. Also, the APIs must support limit/offset/pagination for large datasets.