[query] add AverageBelow function

[yyin-sc]

What this PR does / why we need it:

add AverageBelow function

Special notes for your reviewer:

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

NONE

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

NONE

[codecov]

Codecov Report

Merging #3641 (b64a3f8) into master (29722e7) will decrease coverage by 0.2%.
The diff coverage is n/a.

❗ Current head b64a3f8 differs from pull request most recent head 759c660. Consider uploading reports for the commit 759c660 to get more accurate results

@@           Coverage Diff            @@
##           master   #3641     +/-   ##
========================================
- Coverage    56.3%   56.1%   -0.3%     
========================================
  Files         551     551             
  Lines       61868   61868             
========================================
- Hits        34872   34728    -144     
- Misses      23879   24007    +128     
- Partials     3117    3133     +16     

Flag	Coverage Δ
aggregator	57.1% <ø> (ø)
cluster	∅ <ø> (∅)
collector	58.4% <ø> (ø)
dbnode	60.6% <ø> (-0.4%)	⬇️
m3em	46.4% <ø> (ø)
metrics	19.8% <ø> (ø)
msg	74.1% <ø> (-0.4%)	⬇️

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[robskillington]

LGTM