Expose public exponential histogram mapping functions

[jmacd]

Problem Statement

The OTel-Go Metrics SDK will at some future point incorporate an exponential histogram. This was assembled in #2393 as a reference implementation. At the same time, the OpenTelemetry Collector is looking to support the OTLP v0.11 exponential histogram data point, for example in its logging exporter: open-telemetry/opentelemetry-collector#4642

Proposed Solution

As PR #2393 has become too large to review in one pass, it makes sense to split it into two parts. The mapping functions stand alone and are useful to external OTel compents. I propose to place the sdk/metric/aggregator/exponential/internal/mapping packages shown in #2393 into sdk/metric/aggregator/exponential/mapping, to facilitate this development, in a new PR.

Alternatives

Other paths are possible, possibly outside the sdk/metric hierarchy, however that would seem to create more external surface area.

Additional Context

There is an OTel-Collector-Contrib PR that uses the whole aggregator from #2393, i.e., not only the mapping functions. See open-telemetry/opentelemetry-collector-contrib#6666.