Add host.name label is hostname is present in node.

[kisieland]

Description:
Fixes: #2710

Testing:
I've run go test ./... in the changed exporter

[codecov]

Codecov Report

Merging #2711 (beb54b9) into main (4013aed) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2711   +/-   ##
=======================================
  Coverage   91.57%   91.57%           
=======================================
  Files         447      447           
  Lines       22077    22085    +8     
=======================================
+ Hits        20216    20224    +8     
  Misses       1392     1392           
  Partials      469      469           

Flag	Coverage Δ
integration	69.09% <ø> (ø)
unit	90.50% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
exporter/googlecloudexporter/googlecloud.go	78.70% <100.00%> (+1.70%)	⬆️

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[dashpole]

In an ideal world, we would change the go exporter to look in the node identifier instead of the resource label (also, why does the node resource label exist if the node identifier should be used instead?). However, i'm concerned that it would change behavior for existing OC stackdriver users. If they are setting host.name, we might change the host it is associated with. Given that the change is isolated to the SD exporter, and reverts back to previous behavior, I think this is probably the best thing to do.

[kisieland]

@bogdandrutu PTAL