Expose exponential histogram data structure for public use

[jmacd]

Problem Statement

Following #2501, the OpenTelemetry exponential histogram data type is relatively challenging to implement; OpenTelemetry-Go is a natural place to host common code when it will be used by both the OTel-Go SDK and the Collector. The first part of a solution came in #2502.

The original work in #2393 was carried on at Lightstep and published in otel-launcher-go. Lightstep would like to add support to the OTel Collector statsd receiver to use exponential histogram.

Proposed Solution

The current OpenTelemetry SDK contains the public mapping functions from #2502 in the sdk/metric/aggregator/exponential/mapping package. The proposal is to place a sibling in the exponential package, named sdk/metric/aggregator/exponential/structure that is a pure data structure meant for use by any Golang SDK, receiver or processor that wishes to create or modify expo-histograms.

Alternatives

The OpenTelemetry collector and others could refer directly to the Lightstep implementation here: https://github.com/lightstep/otel-launcher-go/tree/main/lightstep/sdk/metric/aggregator/histogram/structure

However, we think it best to contribute this to otel-go.

[jmacd]

Duplicate of #3170.