[downsampler] Flesh comments in metrics_appender.go

[wesleyk]

What this PR does / why we need it:

Fixes #

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

[codecov]

Codecov Report

Merging #3803 (ddeddc4) into master (ddeddc4) will not change coverage.
The diff coverage is n/a.

❗ Current head ddeddc4 differs from pull request most recent head 523c46e. Consider uploading reports for the commit 523c46e to get more accurate results

@@          Coverage Diff           @@
##           master   #3803   +/-   ##
======================================
  Coverage    56.8%   56.8%           
======================================
  Files         552     552           
  Lines       63077   63077           
======================================
  Hits        35888   35888           
  Misses      23996   23996           
  Partials     3193    3193           

Flag	Coverage Δ
aggregator	63.4% <0.0%> (ø)
cluster	∅ <0.0%> (∅)
collector	58.4% <0.0%> (ø)
dbnode	60.5% <0.0%> (ø)
m3em	46.4% <0.0%> (ø)
metrics	19.7% <0.0%> (ø)
msg	74.4% <0.0%> (ø)

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