Allow option to not emit statsd metrics when reporting a `0` value

We store the statsd metrics envoy emits in a SAAS monitoring tool that charges us based on the number of measurements we store at a particular reporting interval. Some of the metrics we are recording are consistently 0, with occasional spikes when something happens. Here's an example:

I can see the value in using a 0 value to report "nothing is happening", but our monitoring bill has doubled as a result of adopting envoy(!), and continues to grow as we add more clusters, so we'd like to save some money by not recording measurements we can mostly infer from an absence of data.

That said, the zero values were very helpful when were spiking into adopting envoy, as it allowed us to see which metrics we could expect from envoy. Maybe this should be a configuration option in the StatsMatcher proto?