Update auth docs for Collector exporter

[damemi]

Fixes #614
(for now)

This adds more detail around the different ways to authenticate the collector for GCP, de-emphasizing the use of gcloud auth application-default login and instead recommending the GCP service account.

[codecov]

Codecov Report

Merging #621 (e2c6426) into main (f50a6d9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #621   +/-   ##
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  Coverage   66.25%   66.25%           
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  Files          36       36           
  Lines        4264     4264           
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  Hits         2825     2825           
  Misses       1312     1312           
  Partials      127      127           

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[punya]

Consider referencing https://cloud.google.com/docs/authentication/provide-credentials-adc as a canonical doc before summarizing the bits that are most relevant to our use case.

[damemi]

Consider referencing https://cloud.google.com/docs/authentication/provide-credentials-adc as a canonical doc before summarizing the bits that are most relevant to our use case.

sg, added a link at the top