[Metrics SDK] - Add monotonicity property to Sum Aggregation

[lalitb]

Describe your environment

As Sum Aggregation is default mapping for both Counter, and UpDownCounter, it is important to maintain the monotonic property for this aggregation to differentiate the type of value stored. This is required to

• validate the measurement value being recorded.
• Select the correct metric family at the exporter. Eg Prometheus exporter should use gauge for UpDownCounter, and counter for Counter.