Exponential Histogram Prototype

[jsuereth]

This bug tracks work/progress around generating exponential histograms in the Java SDK.

• Add exponential histogram to sdk.metrics.data package
• Add exponential histogram to OTLP exporters
• Create an exponential histogram Aggregator

[jamesmoessis]

@jsuereth I'm going to implement this now that the OTEP seems in it's final form. Would these three items be three separate PRs or one big one?

[jkwatson]

@jsuereth I'm going to implement this now that the OTEP seems in it's final form. Would these three items be three separate PRs or one big one?

Please favor smaller, focussed PRs if at all possible. Thanks!

[jamesmoessis]

Draft PR for the aggregator: #3724

[jamesmoessis]

PR for OTLP marshalers: #3995

Once the above PR gets merged, I think it's safe to close this issue.