[query] Fix groupByNodes to get proper first path

[yyin-sc]

What this PR does / why we need it:

Fix groupByNodes to get proper first path, both in normal cases and special cases like scaleToSeconds(servers.foo-2.pod1.status.500,1)

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 #3640 (af9ea71) into master (29722e7) will decrease coverage by 0.2%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #3640     +/-   ##
========================================
- Coverage    56.3%   56.1%   -0.3%     
========================================
  Files         551     551             
  Lines       61868   61868             
========================================
- Hits        34872   34743    -129     
- Misses      23879   23993    +114     
- Partials     3117    3132     +15     

Flag	Coverage Δ
aggregator	57.1% <ø> (ø)
cluster	∅ <ø> (∅)
collector	58.4% <ø> (ø)
dbnode	60.6% <ø> (-0.3%)	⬇️
m3em	46.4% <ø> (ø)
metrics	19.8% <ø> (ø)
msg	74.3% <ø> (-0.3%)	⬇️

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[robskillington]

LGTM