PoC: allow reservoirs to defer recording the timestamp until storage

[dashpole]

recording the current time can be expensive, so I thought maybe deferring it until after the time-weighted algorithm decided to store it would have a positive impact on performance. But in my testing, it is less than a 1% improvement, which i'm not sure is worth the additional complexity:

Benchmarks are both after #7447 is applied.

                                                                          │ optimizefixed.txt │         defertimestamp.txt         │
                                                                          │      sec/op       │    sec/op     vs base              │
SyncMeasure/NoView/ExemplarsEnabled/Int64Counter/Attributes/0-24                 68.14n ± 11%   65.21n ±  6%       ~ (p=0.093 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Int64Counter/Attributes/1-24                 70.31n ±  9%   73.17n ± 11%       ~ (p=0.180 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Int64Counter/Attributes/10-24                68.20n ±  3%   72.36n ±  6%  +6.11% (p=0.002 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Float64Counter/Attributes/0-24               71.31n ± 12%   67.80n ± 14%       ~ (p=0.589 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Float64Counter/Attributes/1-24               69.35n ± 10%   65.89n ±  4%  -4.98% (p=0.004 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Float64Counter/Attributes/10-24              66.04n ±  6%   65.62n ±  5%       ~ (p=0.310 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Int64UpDownCounter/Attributes/0-24           65.47n ±  7%   68.67n ±  4%       ~ (p=0.180 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Int64UpDownCounter/Attributes/1-24           68.15n ±  7%   66.19n ±  3%       ~ (p=0.132 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Int64UpDownCounter/Attributes/10-24          69.06n ± 12%   67.82n ±  7%       ~ (p=0.093 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Float64UpDownCounter/Attributes/0-24         71.47n ± 10%   66.61n ±  3%       ~ (p=0.180 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Float64UpDownCounter/Attributes/1-24         68.14n ±  7%   66.16n ± 11%       ~ (p=0.589 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Float64UpDownCounter/Attributes/10-24        65.73n ±  4%   67.22n ±  7%       ~ (p=0.589 n=6)
geomean                                                                          68.42n         67.68n        -1.08%

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.2%. Comparing base (19a5a68) to head (ce9849f).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #7711     +/-   ##
=======================================
- Coverage   86.2%   86.2%   -0.1%     
=======================================
  Files        302     302             
  Lines      21971   21977      +6     
=======================================
+ Hits       18950   18954      +4     
- Misses      2640    2642      +2     
  Partials     381     381             

Files with missing lines	Coverage Δ
sdk/metric/exemplar/fixed_size_reservoir.go	94.7% <ø> (ø)
sdk/metric/exemplar/histogram_reservoir.go	90.9% <ø> (ø)
sdk/metric/exemplar/storage.go	100.0% <100.0%> (ø)
...dk/metric/internal/aggregate/filtered_reservoir.go	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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