Move exponential histogram mapping functions into top-level module

[jmacd]

The exponential histogram mapping functions are not SDK-specific and have been until now included in the sdk/metric module, which is being swapped soon. Removing this non-SDK component allows it to be released without disruption as the metrics SDK is replaced.

Part of #2328 because that way the sdk/metric/aggregator directory can be removed completely.

[codecov]

Codecov Report

Merging #3159 (13d8ea0) into main (7aba25d) will decrease coverage by 0.0%.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3159     +/-   ##
=======================================
- Coverage   76.3%   76.2%   -0.1%     
=======================================
  Files        180     180             
  Lines      11992   11992             
=======================================
- Hits        9153    9149      -4     
- Misses      2597    2601      +4     
  Partials     242     242             

Impacted Files	Coverage Δ
data/exponential/mapping/exponent/exponent.go	100.0% <ø> (ø)
data/exponential/mapping/internal/float64.go	100.0% <ø> (ø)
data/exponential/mapping/logarithm/logarithm.go	100.0% <ø> (ø)
sdk/trace/batch_span_processor.go	80.2% <0.0%> (-0.9%)	⬇️
exporters/jaeger/jaeger.go	90.3% <0.0%> (-0.9%)	⬇️

[jmacd]

Note that we agreed to make public exponential-histogram mapping functions available in #2501; the code in sdk/metric/aggregator/exponential/mapping is not meant to disappear with the new metrics SDK.

[jmacd]

If we accept this PR, I will apply the same change of path to #3022 -- the mapping and structure packages are meant to be siblings.

[MrAlias]

I do not think these packages have enough review or use to be included in a stable module just yet.

I think this package could fit into the new metric SDK.

[jmacd]

FWIW, we are using these mapping functions extensively (and exclusively) inside Lightstep.

[jmacd]

enough review

For the record, those reviews were #2982 and #2502.