Support exponential histograms

[MadVikingGod]

The new metric SDK should support exponential histograms.

This would require a way to select them as an aggregation, and the data type in the export path.

The plan as it currently stands is:

• Support Exponential Histograms in metricdata package (Exponential Histogram Datatypes #4165)
• Support Exponential Histograms in exporters (Add Exponetial Histograms to otlp transforms #4222)
• Create an Exponential Histograms internal.Aggregation(Add the Exponential Histogram Aggregator. #4245)

[MadVikingGod]

This will have a number of subtasks:

edit: List moved to top comment to enable task tracking.

[MrAlias]

Discussed in the SIG meeting today: this might better fit in the post-GA metric SDK project. It needs to be verified that this can be added in a backwards compatible manner. If so, work to add it should be postponed until the specification has stabilized this section.

[MrAlias]

Moving to the Metric SDK (GA) project. This needs to be included in the stable release of the SDK as it is a recommended aggregation of the specification.