Cloud Run - Generation1 support

[drandell]

Is your feature request related to a problem? Please describe. Reading the sys directory is not supported under cloud run v1. Therefore when running on cloud run the default_provider function returns the expect from the example. Under cloud run v2 it works as expected returning google compute engine.

Describe the solution you'd like Have a fallback option to try and call the metadata server directly?

Additional context Google docs Using examples/default_creds_id_token.rs and the default_provider function