[query] Return series count header

[arnikola]

What this PR does / why we need it:
Adds a returned header tracking the number of unique series included in the query.

[wesleyk]

LGTM

[codecov]

Codecov Report

Merging #3505 (6007a6e) into master (b2cb0c0) will increase coverage by 0.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #3505     +/-   ##
========================================
+ Coverage    55.9%   56.0%   +0.1%     
========================================
  Files         548     548             
  Lines       61212   61212             
========================================
+ Hits        34218   34339    +121     
+ Misses      23892   23775    -117     
+ Partials     3102    3098      -4     

Flag	Coverage Δ
aggregator	57.4% <ø> (ø)
cluster	∅ <ø> (∅)
collector	54.3% <ø> (ø)
dbnode	60.5% <ø> (+0.2%)	⬆️
m3em	46.4% <ø> (ø)
metrics	19.8% <ø> (ø)
msg	74.6% <ø> (+0.3%)	⬆️

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