allow sending extras when setting up client

[daniel-sanche]

We recently added a feature to allow log commands to send in extras, allowing users to customize certain fields (#129)

@nicoleczhu suggested expanding this by allowing users to set default extras when first setting up the logging client

[oittaa]

It would be great if trace (X-Cloud-Trace-Context header) was included automatically. For example Cloud Run logging is super confusing. The documentation just points all around without leading to any clear solutions and Google-searches direct to some random GitHub repos with people having logging issues.

So please consider adding tracing even in the simplest Cloud Run apps that are basically

import logging
logging.info("message")