FR(metrics): add support for cumulative, non-monotonic sums

[jacobmarble]

@ziomill reported this error in OpenTelemetry.

In OTel->LP translation, for schema telegraf-prometheus-v2, we drop metrics that are sum, cumulative, non-monotonic. However, the OpenTelemetry metrics data model spec indicates that, for Prometheus(-ish) conversion, these should be translated as gauge metrics.