[docs] Add docs for num unique time series

[benraskin92]

What this PR does / why we need it:

Fixes #

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?:

[robskillington]

LGTM

[codecov]

Codecov Report

Merging #2062 into master will decrease coverage by 6.3%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #2062      +/-   ##
=========================================
- Coverage    69.7%   63.3%    -6.4%     
=========================================
  Files        1013     633     -380     
  Lines       87564   61280   -26284     
=========================================
- Hits        61049   38833   -22216     
+ Misses      22152   19111    -3041     
+ Partials     4363    3336    -1027

Flag	Coverage Δ
#aggregator	57.8% <ø> (+10.6%)	⬆️
#cluster	86% <ø> (+35.6%)	⬆️
#collector	48.8% <ø> (+3.7%)	⬆️
#dbnode	63.4% <ø> (+18%)	⬆️
#m3em	51% <ø> (-13.7%)	⬇️
#m3ninx	66.8% <ø> (-3.7%)	⬇️
#m3nsch	100% <ø> (+39.5%)	⬆️
#metrics	17.7% <ø> (ø)	⬆️
#msg	73.6% <ø> (-1.2%)	⬇️
#query	74.6% <ø> (+6.2%)	⬆️
#x	87.2% <ø> (+14.7%)	⬆️

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