chore: add code coverage

[JustinBeckwith]

@stephenplusplus we would need you to go here to set this up :)

https://codecov.io/gh/stephenplusplus/gcp-metadata

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@9686277). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #29   +/-   ##
=========================================
  Coverage          ?   97.77%           
=========================================
  Files             ?        1           
  Lines             ?       45           
  Branches          ?        7           
=========================================
  Hits              ?       44           
  Misses            ?        1           
  Partials          ?        0

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[stephenplusplus]

I think I did it!