Persistent storage

[tgross]

Prometheus mostly punts on the question of long-term storage of metrics. Each Prometheus node stores time series in local files with a LevelDB index. As of this writing it appears the design for remote storage will be to have some kind of generic interface for remote storage over gRPC.

At some point we'll want to have an answer to this question and I want to leave this issue here. In the meantime we'll also be following up on the notion of an example metrics forwarder (ref #13).