Make getBin and scaleChange methods of expoHistogramDataPoint

[MrAlias]

Both functions receive parameters from an expoHistogramDataPoint and are only ever used by other methods of an expoHistogramDataPoint. Make the functions methods of expoHistogramDataPoint so the parameter arguments can be dropped and the functions are scoped to the type they are used for.

[codecov]

Codecov Report

Merging #4451 (ab87e73) into main (9d9b71f) will not change coverage.
The diff coverage is 100.0%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4451   +/-   ##
=====================================
  Coverage   78.9%   78.9%           
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  Files        254     254           
  Lines      20642   20642           
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  Hits       16299   16299           
  Misses      3998    3998           
  Partials     345     345           

Files Changed	Coverage Δ
...metric/internal/aggregate/exponential_histogram.go	96.1% <100.0%> (ø)