[Metrics SDK] Enable Exemplar in Metrics SDK

[lalitb]

The Exemplar implementation is behind feature flag. Need to test the performance with current implementation, do necessary changes if required, and remove the feature flag.

[github-actions[bot]]

This issue was marked as stale due to lack of activity.

[ThomsonTan]

There is also feature gap and currently there seems no way to configure exemplar to the meter provider. Let's use this item to track all the gap to mark exemplar as stable.