Set app log level to logging.DEBUG instead of logging.INFO

Recently I tried using logging to record debugging information in my callbacks via app.logger (see discussion at https://community.plot.ly/t/logging-debug-messages-suppressed-in-callbacks) and found that some debug messages were being ignored, apparently only when they were logged from callbacks.

It turned out that with the default behaviour (i.e. silence_routes_logging=True), debug messages don't get logged after the server is started as Dash sets self.logger.setLevel(logging.INFO) when app.run_server() is called (see https://github.com/plotly/dash/blob/aac493c23366043c3e9612c3f5624ffacbb5d1e9/dash/dash.py#L1162). It's easy to work around, but @ned2 suggested that it might be more sensible for Dash to set its master log level to logging.DEBUG anyway. As an end-user it was not obvious to me why my debug messages were being logged at first and then weren't.

If this would be a sensible change, I'm happy to make a mini-PR.

Inizia in dash/dash.py nel percorso app.run_server() indicato dall’issue, soprattutto nel punto in cui viene impostato il livello del logger. Verifica in che modo silence_routes_logging influisce sui messaggi delle callback e individua il comportamento di logging rilevante da verificare. Il lavoro è completato quando i messaggi di debug delle callback rimangono disponibili dopo l’avvio del server senza richiedere un workaround all’utente finale.