Add Exemplars to metrics generated in aggregate processor

[kkondaka]

Is your feature request related to a problem? Please describe.
Add Exemplars to metrics generated in aggregate processor. See https://opentelemetry.io/docs/specs/otel/metrics/data-model/#exemplars for description about Exemplars

Describe the solution you'd like
Add one of the events used in the Count aggregation for exemplars in the count metric generated
Add min and max events in the Histogram aggregation for exemplars in the histogram metric generated

Describe alternatives you've considered (Optional)
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.