Use sync.Map and atomics for lastvalue aggregations

[dashpole]

Depends on #7474

This applies similar optimizations as #7427 to the last value aggregation.

Changes for last value are contained in 27e1482.

Parallel benchmarks:

                                                                   │   main.txt   │               lv.txt               │
                                                                   │    sec/op    │   sec/op     vs base               │
SyncMeasure/NoView/ExemplarsDisabled/Int64Gauge/Attributes/10-24     264.60n ± 3%   66.46n ± 1%  -74.88% (p=0.002 n=6)
SyncMeasure/NoView/ExemplarsDisabled/Float64Gauge/Attributes/10-24   270.25n ± 4%   69.69n ± 1%  -74.21% (p=0.002 n=6)
geomean                                                               267.4n        68.05n       -74.55%

[codecov]

Codecov Report

❌ Patch coverage is 97.89474% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.8%. Comparing base (0c97dfd) to head (27e1482).

Files with missing lines	Patch %	Lines
sdk/metric/internal/aggregate/atomic.go	84.6%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #7478    +/-   ##
======================================
  Coverage   85.7%   85.8%            
======================================
  Files        287     287            
  Lines      25347   25449   +102     
======================================
+ Hits       21744   21842    +98     
- Misses      3222    3224     +2     
- Partials     381     383     +2     

Files with missing lines	Coverage Δ
sdk/metric/internal/aggregate/aggregate.go	100.0% <100.0%> (ø)
sdk/metric/internal/aggregate/histogram.go	100.0% <100.0%> (ø)
sdk/metric/internal/aggregate/lastvalue.go	100.0% <100.0%> (ø)
sdk/metric/internal/aggregate/sum.go	100.0% <100.0%> (ø)
sdk/metric/internal/aggregate/atomic.go	89.3% <84.6%> (-2.6%)	⬇️

... and 1 file with indirect coverage changes

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