Respect old logging format if given so logs don't change unexpectedly.

[ashb]

If someone is upgrading from any earlier version, their config file with have
log_format in it, even though we have now changed the default to an empty
string.

This also makes respects the colored_console_log config option

If it is specified we should respect it, even though our new default has
changed.

---

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[ramitkataria]

Hey @ashb! It seems like environment variable overrides and TTY check are not working anymore because colors is never None after this was merged.

Here's the overrides and TTY check I'm talking about:

airflow/shared/logging/src/airflow_shared/logging/structlog.py

Lines 404 to 434 in 6f1a20f

	:param colors: Whether to use colors for non-JSON logs. If `None` is passed, then colors are used
	if standard out is a TTY (that is, an interactive session).
	It's possible to override this behavior by setting two standard environment variables to any value
	except an empty string:
	* ``FORCE_COLOR`` activates colors, regardless of where output is going.
	* ``NO_COLOR`` disables colors, regardless of where the output is going and regardless the value of
	``FORCE_COLOR``. Please note that ``NO_COLOR`` disables all styling, including bold and italics.
	:param callsite_parameters: A list parameters about the callsite (line number, function name etc) to
	include in the logs.
	If ``log_format`` is specified, then anything required to populate that (such as ``%(lineno)d``) will
	be automatically included.
	:param namespace_log_levels: Levels of extra loggers to configure.
	To make this easier to use, this can be a string consisting of pairs of ``<logger>=<level>`` (either
	string, or space delimited) which will set the level for that specific logger.
	For example::
	``sqlalchemy=INFO sqlalchemy.engine=DEBUG``
	"""
	if "fatal" not in NAME_TO_LEVEL:
	NAME_TO_LEVEL["fatal"] = NAME_TO_LEVEL["critical"]
	if colors is None:
	colors = os.environ.get("NO_COLOR", "") == "" and (
	os.environ.get("FORCE_COLOR", "") != ""
	or (sys.stdout is not None and hasattr(sys.stdout, "isatty") and sys.stdout.isatty())
	)