The mozlog format allows for a Fields hash of application-specific fields. This is a useful feature for structured logging, and it should be documented how to access it.
As discovered in mozilla.fx-private-relay, this can be populated through the extra parameter:
logger.info(
"Subsystem foo running at foo.example.com:4021",
extra: {"phase": "started", "host": "foo.example.com", "port": 4021})