logging: automatic project detection in logging.NewClient()

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Is your feature request related to a problem? Please describe.
When calling logging.NewClient(), the caller has to specify the parent which is assumed to be a project ID if its type is not explicit.

When running in GCE, GCR, GCF or GAE Standard, the obvious default choice for parent would be the same project in which the code runs. At the same time, in those environments, the project ID can be inferred from the environment (see logging/resource.go).

Describe the solution you'd like
Allow

client, err: = logging.NewClient(ctx, "")

which will set the parent to the project ID in which the code is running.

Describe alternatives you've considered
Callers of the logging library implement their own automatic project ID detection.

Additional context
N/A