Implement sum of squared deviations for exponential histograms in SDK exporter

[dashpole]

Follow-up to #777

See

opentelemetry-operations-go/exporter/metric/metric.go

Line 630 in c7b17b6

// TODO: Implement "setSumOfSquaredDeviationExpHist" for parameter "enableSOSD" functionality.

For fixed-bucket histograms, we assumed each observation occurred at the center of each bucket, and all underflow and overflow observations occurred at the lowest/highest bucket boundary. We could probably implement similar logic for exponential histograms.

[dashpole]

cc @franciscovalentecastro