Optimize histogram reservoir

[dashpole]

This improves the concurrent performance of the histogram reservoir's Offer function by 10x (i.e. 90% reduction).

Accomplish this by:

• Use atomic.Value instead of locking.
• Store references to measurements, rather than measurements themselves for better store/load performance.
• Avoid allocations for measurements using a sync pool.

goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/sdk/metric/exemplar
cpu: Intel(R) Xeon(R) CPU @ 2.20GHz
                           │  main24.txt   │              new24.txt              │
                           │    sec/op     │    sec/op     vs base               │
FixedSizeReservoirOffer-24    254.8n ±  4%   246.6n ±  8%        ~ (p=0.394 n=6)
HistogramReservoirOffer-24   257.40n ± 10%   24.35n ± 12%  -90.54% (p=0.002 n=6)
geomean                       256.1n         77.50n        -69.74%

                           │  main24.txt  │             new24.txt              │
                           │     B/op     │    B/op     vs base                │
FixedSizeReservoirOffer-24   0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=6) ¹
HistogramReservoirOffer-24   0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=6) ¹
geomean                                 ²               +0.00%               ²
¹ all samples are equal
² summaries must be >0 to compute geomean

                           │  main24.txt  │             new24.txt              │
                           │  allocs/op   │ allocs/op   vs base                │
FixedSizeReservoirOffer-24   0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=6) ¹
HistogramReservoirOffer-24   0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=6) ¹
geomean                                 ²               +0.00%               ²

Depends on #7441. Marking as a draft until that is merged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.7%. Comparing base (0c97dfd) to head (77db4ee).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #7443   +/-   ##
=====================================
  Coverage   85.7%   85.7%           
=====================================
  Files        287     287           
  Lines      25347   25359   +12     
=====================================
+ Hits       21744   21757   +13     
+ Misses      3222    3220    -2     
- Partials     381     382    +1     

Files with missing lines	Coverage Δ
sdk/metric/exemplar/histogram_reservoir.go	92.0% <100.0%> (-1.4%)	⬇️
sdk/metric/exemplar/storage.go	96.6% <100.0%> (-3.4%)	⬇️

... and 2 files with indirect coverage changes

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