[query] when using unless op do not remove metric name

[gediminasgu]

What this PR does / why we need it: PromQL compatibility fix. When using unless operator in the query it should not remove metric names from the result.

Sample query: http_requests{group="canary"} unless http_requests{instance="0"}

The result before the fix: {group="canary", instance="1", job="api-server"}

The result after the fix: http_requests{group="canary", instance="1", job="api-server"}

Fixes #

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[bot]]

Codecov Report

Merging #2418 (704afa0) into master (b12af4d) will increase coverage by 0.0%. Report is 1336 commits behind head on master. The diff coverage is n/a.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2418   +/-   ##
======================================
  Coverage    71.5%   71.5%           
======================================
  Files        1063    1063           
  Lines       93410   93408    -2     
======================================
+ Hits        66825   66827    +2     
  Misses      22093   22093           
+ Partials     4492    4488    -4     

Flag	Coverage Δ
aggregator	76.4% <ø> (+<0.1%)	:arrow_up:
cluster	84.8% <ø> (-0.1%)	:arrow_down:
collector	82.8% <ø> (+35.3%)	:arrow_up:
dbnode	79.0% <ø> (+<0.1%)	:arrow_up:
m3em	74.4% <ø> (ø)
m3ninx	73.3% <ø> (+<0.1%)	:arrow_up:
m3nsch	51.1% <ø> (ø)
metrics	17.2% <ø> (-17.7%)	:arrow_down:
msg	75.1% <ø> (-0.2%)	:arrow_down:
query	67.4% <ø> (-0.1%)	:arrow_down:
x	81.8% <ø> (ø)

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