Only metrics called 'count' will be summed

See graphite/storage-aggregation.conf

By default metrics are averaged when they are aggregated up. This is not
correct for metrics that represent a count. In these cases they should
be summed. To do this the metric must be called 'count'.
Set xFilesFactor to zero for statsd timers

We have very low throughput on the performance platform which means that
timers often get aggregated to null because fewer than 10% of the values
are non-null. The risk with doing this change is that we will loose visibility
of flakeyness in metrics (ie. a metric stops sending for a few seconds,
it will still be aggregated to a number if there is any number in it).
That is why it is only being applied to statsd timers.
Merge pull request #537 from alphagov/count-status-codes

Only metrics called 'count' will be summed