GOOGLE_APPLICATION_CREDENTIALS not picking up

[JensJI]

Hi

Thanks for some great work!

I'm using version v2.0 on GKE.

I tried parsing the JSON contents into the environment variable GOOGLE_APPLICATION_CREDENTIALS but it didn't work.
The logs mentioned that it tried to read it as a file (so instead of the contents it thought it was a path to a file).

But the docs says that GCLOUD_CREDENTIALS_FILE is for providing a filename and GOOGLE_APPLICATION_CREDENTIALS is for providing file contents.

I searched the source code and found out, that I could provide the JSON contents through the environment variable GCLOUD_JSON_KEYFILE_STRING.
That worked.

Have I misunderstood something or is there an issue?

I could make a PR to the docs if it could help.
I could maybe make a PR to the code, but not sure I am skilled enough.

Best regards from a happy user!