[Metric SDK] Delta metric collection for Asynchronous Aggregation storage

[lalitb]

Refer to data model for delta and cumulative metric data points.

This is dependent on #1232 which implements Asynchronous storage.

Implement AsyncMetricStorage::Collect to fetch the delta metric data from the hashmap of aggregators and return it as a callback/ or span/list of metric data.
The aggregation-hashmap maintains the delta aggregation by default, which needs to be cleaned up after collection.

[github-actions]

This issue was marked as stale due to lack of activity. It will be closed in 7 days if no furthur activity occurs.

[lalitb]

#1334 fixes this.