(https://github.com/ruby/logger/pull/103) `Logger#with_level` was recently added to enable configuring a `Logger`'s level for the duration of a block. However, the configured level is always tied to the currently running `Fiber`, which is not always ideal in applications that mix `Thread`s and `Fiber`s. For example, Active Support has provided a similar feature (`ActiveSupport::Logger#log_at`) which, depending on configuration, can be isolated to either `Thread`s or `Fiber`s. This commit enables subclasses of `Logger` to customize the level isolation. Ideally, it will enable replacing most of Active Support's `#log_at`, since both methods end up serving the same purpose. https://github.com/ruby/logger/commit/dae2b832cd