chore: add some unit test coverage for credentials;metrics;env/fs packages

[markphelps]

First pass at increasing UT coverage

Re: #3942

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.

Please upload report for BASE (v2@656ecfa). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...nternal/storage/environments/fs/testing/testing.go	81.25%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##             v2    #3943   +/-   ##
=====================================
  Coverage      ?   53.51%           
=====================================
  Files         ?      117           
  Lines         ?    10754           
  Branches      ?        0           
=====================================
  Hits          ?     5755           
  Misses        ?     4614           
  Partials      ?      385           

Flag	Coverage Δ
unittests	53.51% <83.33%> (?)

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