Add support for saving local samples as Exponential Histograms

[ndipebot]

Exponential Histogram for saving raw sample.

Use the exponential histogram proto defined here: https://github.com/open-telemetry/oteps/blob/main/text/0149-exponential-histogram.md

Google Scheme is used for histogram definition from config.

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[pwittrock]

/lgtm
/approve

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