exphist: replace min, max, sum, and count with atomics

[dashpole]

This is the first PR towards a lockless fast-path for the exponential histogram aggregation. It just replaces use of min, max, sum and counts with atomic types.

You can see the full set of planned changes in main...dashpole:opentelemetry-go:lockless_exphist_ai. The implementation is largely based on #7535 (which I implemented by hand), but with help from an AI to break it down into smaller PRs, and simplify aspects of the design.

Part of #7796

[codecov]

Codecov Report

❌ Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.6%. Comparing base (82d57cc) to head (3d043ce).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...metric/internal/aggregate/exponential_histogram.go	96.1%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #8025     +/-   ##
=======================================
- Coverage   81.6%   81.6%   -0.1%     
=======================================
  Files        304     304             
  Lines      23452   23446      -6     
=======================================
- Hits       19147   19136     -11     
- Misses      3923    3928      +5     
  Partials     382     382             

Files with missing lines	Coverage Δ
...metric/internal/aggregate/exponential_histogram.go	98.6% <96.1%> (-0.1%)	⬇️

... and 4 files with indirect coverage changes

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