Add `user_attrs` to the list of parameters that you can plot

Motivation

My use case was optimising a LightGBM model where I use early stopping to determining the number of boosting rounds. I can record the number of estimators in a trial using trial.set_user_attr but I haven't figured out a way to plot this using the usual optuna.visualisation package.

Is there a way how to do this? If not could this be added? What would be the easiest way to add this and I maybe could give it a go.

Thanks!

Description

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