[Metric SDK] Delta metric collection for Synchronous Aggregation storage

[lalitb]

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

Implement SyncMetricStorage::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.