First pass at fixing NPEs while ingesting statsd metrics

Currently, we make an assumption that all the sub-metrics within a pre-aggregated statsd metric always exist. We are seeing data right now which makes us believe that this assumption is wrong. It might cause NPEs if some of sub-metrics are missing.
This is a quick fix, which will set the sub-metrics to 0s if they do not exist. I completely understand that this is not a permanent fix, but making changes to support non-existence of sub-metrics is a bit contrived and I will start working on it. Just getting this out there, so that we can continue ingesting statsd metrics without interruption.