Prometheus exporter should convert non-monotonic sums to gauges

[dashpole]

The current implementation of Sum export in the prometheus exporter does not differentiate between monotonic and non-monotonic sums.

The prometheus compatibility spec for sums says: If the aggregation temporality is cumulative and the sum is non-monotonic, it MUST be converted to a Prometheus Gauge..

[brettimus]

I'll pick this up!

[ocelotl]

@brettimus I assigned this issue to you, thanks ✌️