Stagger timestamps in exact aggregator tests.

[punya]

Fixes #1559.

[codecov]

Codecov Report

Merging #1568 (8f36512) into main (b94cd4b) will decrease coverage by 0.2%.
The diff coverage is 0.0%.

@@           Coverage Diff           @@
##            main   #1568     +/-   ##
=======================================
- Coverage   78.2%   78.0%   -0.3%     
=======================================
  Files        127     127             
  Lines       6577    6600     +23     
=======================================
- Hits        5149    5148      -1     
- Misses      1183    1205     +22     
- Partials     245     247      +2     

Impacted Files	Coverage Δ
sdk/metric/aggregator/exact/exact.go	84.6% <0.0%> (-9.9%)	⬇️
attribute/type_string.go	10.0% <0.0%> (-23.4%)	⬇️
metric/instrumentkind_string.go	10.0% <0.0%> (-23.4%)	⬇️
metric/number/kind_string.go	16.6% <0.0%> (-16.7%)	⬇️
sdk/export/metric/exportkind_string.go	0.0% <0.0%> (ø)