class Logger

- +nil+: the logger uses '%Y-%m-%dT%H:%M:%S.%6N'.
{Time#strftime}[https://docs.ruby-lang.org/en/master/Time.html#method-i-strftime].
- A string suitable for use as a format for method

Argument +datetime_format+ should be either of these:

Sets the date-time format.

def datetime_format=(datetime_format)
  @default_formatter.datetime_format = datetime_format
end