formatter

        # Logger.formatter

(from ruby core)
---

Logging formatter, as a `Proc` that will take four arguments and return
the formatted message. The arguments are:

`severity`
:   The Severity of the log message.
`time`
:   A Time instance representing when the message was logged.
`progname`
:   The #progname configured, or passed to the logger method.
`msg`
:   The *Object* the user passed to the log message; not necessarily a
    String.


The block should return an Object that can be written to the logging
device via `write`.  The default formatter is used when no formatter is
set.