MediaWiki generally only uses four levels:
* 'debug': Verbose information, e.g. during local dev, CI, and
in "Verbose" mode in production via WikimediaDebug.
* 'info': Diagnostics. Enabled for most channels even for production
traffic, though typically hidden in Logstash along with 'debug',
and instead reviewed in combination with other messages from the
same request when the request is investigated for a different reason.
* 'warning': Diagnostics,. Typically hidden in Logstash, except for
dashboards for component's own maintainers, to assess system health.
Warnings are rare but expected to some degree at scale.
* 'error': Something has gone wrong that should in theory not happen.
Typically included even in system-wide dashboards and sometimes used
for alerting.
Afaik we have no meaning associated with 'notice'. At WMF I found
no dashboards selecting it, no alerts using it, and no channels used
it as the threshold for including or discarding messages.
Instead, lower them to info(), or raise them to warning().
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