Restore the exponential mapping functions

[jmacd]

The exponential mapping functions that had been originally reviewed in #2502 (matching specification v1.8) and modified in #2982 (matching specification v1.13). This code is unmodified from its previous state, only the package names change.

The proposed new path for this code is sdk/metric/metricdata/exponential/mapping.

Fixes #3169

The corresponding structure would be its sibling in sdk/metric/metricdata/exponential/structure (part of #3170).

Part of #2966.

[codecov]

Codecov Report

Merging #3243 (ad89d96) into main (aca054b) will increase coverage by 0.2%.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3243     +/-   ##
=======================================
+ Coverage   77.3%   77.6%   +0.2%     
=======================================
  Files        159     162      +3     
  Lines      11184   11295    +111     
=======================================
+ Hits        8651    8766    +115     
+ Misses      2335    2331      -4     
  Partials     198     198             

Impacted Files	Coverage Δ
...etricdata/exponential/mapping/exponent/exponent.go	100.0% <100.0%> (ø)
...metricdata/exponential/mapping/internal/float64.go	100.0% <100.0%> (ø)
...ricdata/exponential/mapping/logarithm/logarithm.go	100.0% <100.0%> (ø)
sdk/trace/batch_span_processor.go	81.9% <0.0%> (+1.7%)	⬆️

[jmacd]

There are now slight upstream changes (e.g., an improved comment). https://github.com/lightstep/go-expohisto