Add more unit tests for creating metrics, fix bug related to unmarshalling flags

[IsaacPD]

Related: #5157
Merge after: #5168

Description

• Fix bug by saving the flag value rather than the entire flag object
• Assert that cloud monitoring exporter is not created if no keys/project id are provided
• Compare the output created by the stdout exporter to what we expect should happen given the test skaffoldMeter structs provided.

[codecov]

Codecov Report

Merging #5169 (ae04b18) into master (531adb6) will increase coverage by 0.50%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master    #5169      +/-   ##
==========================================
+ Coverage   71.91%   72.42%   +0.50%     
==========================================
  Files         385      385              
  Lines       13737    13735       -2     
==========================================
+ Hits         9879     9947      +68     
+ Misses       3128     3062      -66     
+ Partials      730      726       -4     

Impacted Files	Coverage Δ
pkg/skaffold/instrumentation/meter.go	71.22% <93.33%> (+47.10%)	⬆️
pkg/skaffold/docker/image.go	80.93% <0.00%> (+1.39%)	⬆️

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