(feature request): read project id from metadata

[ilteoood]

At the moment, the project id is a required parameter.
In environments like Google App Engine standard, the project id can be extracted from the credentials file or from the GOOGLE_CLOUD_PROJECT.

Can you consider to make it optional?
I would be more than happy to contribute to it.

[abdolence]

Hey,

Google Cloud SDK already have a method to detect project ID from environment variable, service account key file, etc.
https://github.com/abdolence/gcloud-sdk-rs/blob/374b86ffda52c2d133c04259346b3a10063d3724/gcloud-sdk/src/api_client.rs#L151
So, this is simple to implement if needed and I'm gladly accept a PR if you want to give a try :)
Something like for_default_project_id() that just wraps call GoogleEnvironment::detect_google_project_id() and calls new with it. It will be async though, since detection in Google servers and environments is a network call.