[aggregator] Cap length of sample for reusal

[vdarulis]

What this PR does / why we need it:

Reduce memory usage of sample lists that were used by very high-volume timers.
Really should just move to an array list or something, but that is more involved and needs perf testing.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

Does this PR require updating code package or user-facing documentation?:

[codecov]

Codecov Report

Merging #3536 (04883d7) into master (d051ea4) will decrease coverage by 0.0%.
The diff coverage is 0.0%.

@@           Coverage Diff            @@
##           master   #3536     +/-   ##
========================================
- Coverage    55.9%   55.8%   -0.1%     
========================================
  Files         549     549             
  Lines       61434   61437      +3     
========================================
- Hits        34366   34307     -59     
- Misses      23966   24012     +46     
- Partials     3102    3118     +16     

Flag	Coverage Δ
aggregator	57.1% <0.0%> (-0.1%)	⬇️
cluster	∅ <ø> (∅)
collector	54.3% <ø> (ø)
dbnode	60.2% <ø> (-0.1%)	⬇️
m3em	46.4% <ø> (ø)
metrics	19.8% <ø> (ø)
msg	73.9% <ø> (-0.7%)	⬇️

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