[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.

[euroelessar]

@lalitb Are you open to accept changes to implement exponential histogram, given that it's now a stable feature?
https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/data-model.md#exponentialhistogram

[lalitb]

@euroelessar Yes, the contributions are welcome. Thanks for volunteering. Let me know if you want me to assign it to you ?

[euroelessar]

@lalitb Yeah. I don't have a lot of throughput in the next month or so, but should be able to slowly produce some changes.
I have started with an implementation of an underlying adapting circular buffer (as suggested by the spec).

[lalitb]

@euroelessar - do you plan to incorporate the remaining changes for this feature? It's not urgent, but but just in case someone want to pick up the further changes :)

[euroelessar]

@lalitb yep! I need to update tests, implementation itself is overall ready, should be able to send it within a week or so

[lalitb]

@euroelessar Let us know if you need any help on getting this ready? Alternatively, if someone else could take it from here, since you've already set up the foundational data structures. :)

[ThomsonTan]

@euroelessar is there any update on this?

[luan]

This would be super cool if finished, @euroelessar are you still working on it?