[Metrics SDK] Add support for Exponential Histogram Aggregation

[lalitb]

Currently, metrics SDK implementation support following aggregation:

opentelemetry-cpp/sdk/include/opentelemetry/sdk/metrics/instruments.h

Lines 30 to 37 in 88e23ad

	enum class AggregationType
	{
	kDrop,
	kHistogram,
	kLastValue,
	kSum,
	kDefault
	};

The default Histogram aggregation is implemented as described in the data model: https://github.com/open-telemetry/opentelemetry-specification/blob/90e5022a846a65600b7ad7e8d262678a83f76191/specification/metrics/datamodel.md#histogram

The data model also adds (experimental) support for Exponential histogram aggregation - https://github.com/open-telemetry/opentelemetry-specification/blob/90e5022a846a65600b7ad7e8d262678a83f76191/specification/metrics/datamodel.md#exponentialhistogram

This support needs to be implemented in the SDK.

[lalitb]

Moving it out of RC, as the current specs is still not stable, and the complexities involved in implementing this.

[github-actions]

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