Add a custom log level for user messages

[rwb27]

Currently, we use Thing.logger.info() to relay messages to the user from long-running actions. It would be nice to add a log level between INFO and WARNING that is used for messages that ought to be displayed in the front-end.

This would probably entail making Thing.logger return a subclass of logging.Logger with a new method for the custom level.

It would additionally be helpful to add a function that returns a subclassed Logger pointing at labthings_fastapi.logs.THING_LOGGER so it can be used by "thing-adjacent" code that's not part of a Thing.

[rwb27]

I'm going to make a start on a PR to implement something like this. However, I'm going to flag that the logging docs come with a warning that libraries defining custom loglevels can be problematic, because it might lead to libraries defining conflicting new levels.

I think in this limited case it's probably OK, because LabThings has become more of a framework than a library. For what it's worth, the logging module strongly suggests that we don't attempt to return our own Logger subclass: instead, we should use getLoggerClass and setLoggerClass to ensure all loggers have the new method. I think this is due to the way logger objects get cached: instantiating one ourselves would break the cache.

We didn't particularly debate a name for the new level. How would you feel about USER? I will list some suggestions below but that's my favourite I think:

• USER
• INTERFACE
• MESSAGE
• UPDATE
• STATUS