Make exponential histograms more available

[jack-berg]

We have an exponential histogram implementation but because exponential histograms aren't yet stable in the metrics spec, the implementation is obscure / difficult to use. This issue is tracking steps to make histograms easier to use, even while they remain experimental. If they're easier to use, we can get feedback that may help stabilize the specification.

• Align exponential histogram implementation with specification Tighten up exponential histogram implementation #4536
• MetricReader can influence default aggregation by instrument kind MetricReader and MetricExporter can determine default aggregation by instrument #4472
• MetricExporter can influence default aggregation of PeriodicMetricReader
• Add autoconfigure option for using exponential histogram aggregation as default aggregation for otlp metric exporters. Implementation of spec PR #2619.