Prometheus exporter: Non-monotonic sum should be converted to a prometheus gauge

[dashpole]

From the Prometheus spec for OTel sums:

OpenTelemetry Sums follows this logic:

• If the aggregation temporality is cumulative and the sum is monotonic, it MUST be converted to a Prometheus Counter.
• If the aggregation temporality is cumulative and the sum is non-monotonic, it MUST be converted to a Prometheus Gauge.
• If the aggregation temporality is delta and the sum is monotonic, it SHOULD be converted to a cumulative temporality and become a Prometheus Sum. The following behaviors are expected:
  • The new data point type must be the same as the accumulated data point type.
  • The new data point's start time must match the time of the accumulated data point. If not, see detecting alignment issues.
• Otherwise, it MUST be dropped.

The current implementation converts non-monotonic sums to counters. Instead, these should be converted to gauges.