Create Summary Aggregate

[mplachter]

As a user, I would like summary aggregates to get calculated.

[SpangleLabs]

I would love this functionality too! A summary is just a counter and a gauge, and currently both of those are summed, so why aren't summaries simply summed together also?
(though #65 would present an issue there)

At the moment, I'm actually seeing the first summary push works, then the second one clears it and results in the metrics endpoint failing with:

An error has occurred during metrics encoding:

MetricFamily has no metrics: name:"e2erunner_time_taken_seconds" type:SUMMARY

and then the third push works, fourth breaks it again, etc.