Dogstream aggregate should sum counters

[conorbranagan]

Right now if you submit a counter from dogstream it just sends that value multiple times for the same timestamp. It should sum across before sending the metric.

[charles-dyfis-net]

To be a bit more clear --

checks.datadog.Dogstream._aggregate() takes the median value for each second, regardless of whether a value is a counter or a gauge.

Thus, a single packet sent to datadog can include multiple values for a counter (at different seconds); however, all but one value per second will be discarded -- whereas for a counter, the correct behavior is clearly to sum them.