storage latency stats break stats-provider interface

[warner]

The dictionary of storage latency statistics that I added recently causes Violation exceptions when the stats-gatherer tries to retrieve stats. The problem is that the RIStatsProvider interface requires numerical values, whereas the latency stats are all clumped together into a dictionary.

We could change the interface, or flatten the latency stats into the top-level dict.

[warner]

Fixed, in af4e4dc8478ef394, by flattening the latency stats. They now appear with keys like storage_server.latencies.readv.90_percentile .