Stats: search latency implementation scaffolding

Created by: rvantonder

This adds the the functions and types for tracking search latency. It does not record latency stats, which will happen in a future PR.

The way logging will work is by doing, e.g., something like

logMsg := fmt.Sprintf(`{"durationMs": %s}`, strconv.FormatInt(int64(durationMs), 10))
LogBackendEvent(0, "search.latencies.literal", logMsg)