Exporting isMonotonic flag to exporters

[legendecas]

I'm wondering if we should indicate isMonotonic in SingularMetricData. We have access to the data anyway, it is part of the proto, and it simplifies the transformation to OTLP.

Originally posted by @pichlermarc in #2990 (comment)

@pichlermarc you said you would like to work on this so I'm assigning this to you :).

[pichlermarc]

This was a bit more work than I expected it to be. 😅

When I made that comment I forgot that there is only the SingularDataPoint interface that would have had to contain an optional isMonotonic field. Using that to distinguish between Sums and Gauges seemed very fragile, so to reduce the complexity in the exporter transformations, I had to split it up into a SumDataPoint (which contains isMonotonic) and a GaugeDataPoint (which does not contain it). Changes can be found in #3079