Remove the expoHistogramValues type

[MrAlias]

Cleanup the exponential histogram aggregate.

The only use of the expoHistogramValues is it being embedded in the expoHistogram. Just explicitly define the expoHistogramValues fields for expoHistogram given that is their only use.

[codecov]

Codecov Report

Merging #4450 (ae5ee59) into main (a5ff7af) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4450     +/-   ##
=======================================
- Coverage   78.9%   78.9%   -0.1%     
=======================================
  Files        254     254             
  Lines      20650   20642      -8     
=======================================
- Hits       16307   16297     -10     
- Misses      3998    4000      +2     
  Partials     345     345             

Files Changed	Coverage Δ
...metric/internal/aggregate/exponential_histogram.go	96.1% <100.0%> (-0.2%)	⬇️

... and 1 file with indirect coverage changes