Exponential histogram: defer computing count until collect

[dashpole]

Tiny part of #7535

It doesn't make much of a difference now, since we are bound by lock contention, but it matters once we move to atomics.

                                                                                  │  main.txt   │           defercount.txt           │
                                                                                  │   sec/op    │    sec/op     vs base              │
SyncMeasure/NoView/ExemplarsDisabled/ExponentialInt64Histogram/Attributes/0-24      300.9n ± 7%   282.8n ± 10%       ~ (p=0.394 n=6)
SyncMeasure/NoView/ExemplarsDisabled/ExponentialInt64Histogram/Attributes/1-24      300.0n ± 8%   307.5n ±  3%       ~ (p=0.394 n=6)
SyncMeasure/NoView/ExemplarsDisabled/ExponentialInt64Histogram/Attributes/10-24     301.4n ± 5%   300.7n ±  5%       ~ (p=0.485 n=6)
SyncMeasure/NoView/ExemplarsDisabled/ExponentialFloat64Histogram/Attributes/0-24    286.4n ± 4%   284.8n ±  3%       ~ (p=0.420 n=6)
SyncMeasure/NoView/ExemplarsDisabled/ExponentialFloat64Histogram/Attributes/1-24    303.8n ± 3%   294.3n ±  2%  -3.11% (p=0.006 n=6)
SyncMeasure/NoView/ExemplarsDisabled/ExponentialFloat64Histogram/Attributes/10-24   297.9n ± 4%   291.6n ±  3%  -2.10% (p=0.024 n=6)
geomean                                                                             298.3n        293.5n        -1.62%

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.2%. Comparing base (30f1b6c) to head (483d022).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #7702     +/-   ##
=======================================
- Coverage   86.2%   86.2%   -0.1%     
=======================================
  Files        302     302             
  Lines      21985   21991      +6     
=======================================
+ Hits       18963   18968      +5     
- Misses      2642    2643      +1     
  Partials     380     380             

Files with missing lines	Coverage Δ
...metric/internal/aggregate/exponential_histogram.go	97.3% <100.0%> (+<0.1%)	⬆️

... and 1 file with indirect coverage changes

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