Just discovered some changes in logging that to my knowledge weren't documented:
The new Log package isn't autoloaded, only in core/base.php the logger() method will load the package. That means that if that method doesn't get called before you use \Log in you own classes, you'll get a Class 'Log' not found error (which ironically does get logged, due to that being outputted by the logger() method).
This is different behaviour from 1.4 and there is as far as I know no documentation about this? Quick fix is of course adding 'Log' to the autoload package array in your config.php
Another small issue is the order of log levels, previously the Debug level would not output Info messages, while the Info level would output Debug messages. As it is documented everywhere:
Fuel::L_NONE
Fuel::L_ERROR
Fuel::L_WARNING
Fuel::L_DEBUG
Fuel::L_INFO
Fuel::L_ALL
Monolog uses a different order and switches Debug and Info in that list. So L_DEBUG will now output Info messages, while L_INFO won't output Debug messages.
Not sure if I should mark these as issues in the repos, but for future reference I thought it would be helpful for others :)
Correct, you need to add it to always_load, as documented in the change log.
I missed the fact that DEBUG and INFO is swapped, but to me it's logical that you display INFO before you display DEBUG, so in a sense Monolog is right and Fuel has been wrong.