Prometheus Summary needs delta calculation on "sum" metric

[ardias]

According to the slack threads below, the "sum" component of the summary metric needs to be "delta calculated" in order for NRQL to be able to calculate averages on Prometheus summary values.

https://newrelic.slack.com/archives/CME11J4PL/p1602875630066900
https://newrelic.slack.com/archives/CP2EWSX7C/p1603384162298100

Summary must be handled like Histogram: calculate delta on "sum" and only send value if we can in fact calculate it, i.e., it's not the first value we received.

See this PR for the implementeation on the telemetry sdk emitter: newrelic/nri-prometheus#101

[noly]

Thank for your feedback @ardias, I will start to work on this now.

I will let you know when is solved.