Document logging filter

[skearns64]

#5036 added the ability to filter auth headers from the logs, but I don't believe it allows the admin to customize what gets filtered.

This issue tracks allowing the admin to configure which headers are filtered out from the logs (e.g. as a setting in the kibana.yml).

If this is already possible, then :) and this issue can track adding the mechanism to the docs.

[spalger]

This is actually possible. The logging.filter setting can be set to keys that should be removed from anything that gets logged. The filtering of the authorization header is accomplished by the default setting:

--logging.filter.authorization=remove

This causes the "authorization" key to be removed from any object logged by kibana. No matter where it appears, the logger will recurse through nested objects and remove any authorization key.

[palecur]

PR #6451 is up to address this for 4.5 and 4.3. PR #6452 backports the relevant bit to 4.4.