Don't disable existing loggers when initialising Celery.

Prior to this change, the Celery logging did not include any of the logging
calls within MediaGoblin because `disable_existing_loggers` defaulted to True.
This was unhelpful and inconsistent with the logging behaviour of the web process.

The MediaGoblin web process sets logging output based on the configuration in
paste.ini. This is loaded by the `paster` program, rather than MediaGoblin
itself.

The MediaGoblin celery process manually loads its logging config from paste.ini,
but previously defaulted to `disable_existing_loggers=True`, meaning that none
of the application logging flowed through unless the logger was explicitly added
to paste.ini.

docs/source/siteadmin/relnotes.rst

@@ -50,6 +50,7 @@ This chapter has important information about our current and previous releases.
 - Fix test suite on Debian 11 (Ben Sturmfels)
 - Remove reference to jsonschema.compat no longer available upstream (Marco Pessotto)
 - Fix images being marked as failed after Celery restart [#5608] (Ben Sturmfels)
+- Fix app logging calls not showing in Celery logs (Ben Sturmfels)
 
 
 0.11.0

mediagoblin/init/celery/from_celery.py

@@ -45,7 +45,7 @@ def setup_logging_from_paste_ini(loglevel, **kw):
         raise OSError('{} does not exist. Logging can not be set up.'.format(
             logging_conf_file))
 
-    logging.config.fileConfig(logging_conf_file)
+    logging.config.fileConfig(logging_conf_file, disable_existing_loggers=False)
 
     hook_runall('celery_logging_setup')