…leep (#1547)

chore: No sleep on initial attempt in exponential backoff implementation

It is unintuitive that the initial attempt in the exponential backoff
loop sleeps. This can lead to subtle bugs in future call sites.

This patch refactors the exponential backoff to begin sleeping on the
2nd iteration so requests can be done in a single for loop.

…adata.py (#1545)

* fix(metadata): Use exponential backoff retry logic for GCE metadata server access

)

* implement base classes for credentials and request sessions

* remove public methods apply and before_request from base credentials

* move MTLS logic back to requests

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* minor fix

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* address PR comments

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* address PR comments

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* chore: Refresh system test creds.

* revert copyright date

* fix import statements order

---------

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Carl Lundin <108372512+clundin25@users.noreply.github.com>
Co-authored-by: Carl Lundin <clundin@google.com>

* feat: implement async oauth2 credentials

* minor cleanup

* inherit base credentials class in async credentials

* fix whitespace

* implement builder class for oauth 2.0 credentials

* feat: implement async static credentials

* revert implementing oauth2 credentials

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* update values used in tests

* update the exception raised for static refresh

* add async anonymous credentials

* update docstrings

* address PR comments

* chore: Refresh system test creds.

* fix lint issues

* add test coverage

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Co-authored-by: Carl Lundin <clundin@google.com>

Credentials expects `aws_security_credentials_supplier`, not `aws_security_token_supplier`

See #1565
for more information.

* chore(main): release 2.33.0