Add logging to create_access_token_credential

[stepancheg]

This is a small feature request. Feel free to close if not important/already planned/too hard/etc.

Is your feature request related to a problem? Please describe.

When auth doesn't work, error message is not always helpful: some error from HTTP service.

called `Result::unwrap()` on an `Err` value: CredentialError { is_retryable: false, source: "Failed to fetch token. {\n  \"error\": \"invalid_grant\",\n  \"error_description\": \"reauth related error (invalid_rapt)\",\n  \"error_uri\": \"https://support.google.com/a/answer/9368756\",\n  \"error_subtype\": \"invalid_rapt\"\n}" }thread 'tests::test2' panicked at xxx/src/lib.rs:9:55:

Turns out my local gcloud login expired. But at the time I was not even sure, that it actually used auth provided by gcloud command (file ~/.config/gcloud/application_default_credentials.json).

Describe the solution you'd like

Add debug statements how authentication was obtained, for example:

logging::debug!("ADC path found using well known path: {}", adc_path);
...
logging::debug!("Calling URL https://... to obtain token ...")

Ideally, produce error message not just with final http response, but also with information

• why this URL was called
• suggestion to re-run gcloud auth login gcloud auth application-default login if something is expired and needs renewing

(I am a new user of GCP, so don't know well how it works.)

Describe alternatives you've considered

Invoke gcloud auth application-default print-access-token command directly, or call https://metadata.google.internal/... directly, as it is easier to debug.

[coryan]

Thanks for the detailed bug report. Tagging #838 because there is some overlap with that feature request.