Search specification of for discovery interface.

The format of the query string will be left entirely up to individual `DiscoveryIO` implementations.

We'll be changing the way `ElasticIO` works to just parse the entire query rather than associated the term to the RAW_METRIC_NAME field. But this is a good starting point.
I believe we want `RAW_METRIC_NAME` to preserve compatibility with the current tests and previous search implementation.

We'll probably change that.