Option to make exponential histogram default aggregation for OTLP exporter

[jack-berg]

Once exponential histograms are a stable part of the metrics SDK, they'll likely be a better default histogram aggregation option than explicit bucket histograms, since they can dynamically adjust to best capture the distribution of measurements that has been recorded.

We should add an option to the OTLP Metrics Exporter to change its default histogram aggregation from explicit bucket to exponential.

Discussed in #2429.