Trigger warning if configuration source is providing an unknown property

[todvora]

Configuration files idea: should we warn if you configure a property that we don't know? Opensearch does this and it helps to realize you are doing something that isn't working. Last time I've seen an usage of disable_security in datanode, but the actual property is called insecure_startup . By triggering an error or at least warning, we'd tell the user that they are trying to do something that we don't understand. It would remove some portion of trial and error during configuration.

[bernd]

We talked about this topic outside of GitHub

The summary is that (in Graylog) we want to emit a log warning when a configuration file contains unknown properties. We don't want to throw an error and abort the server startup. Users might have older config settings in their config which are currently ignored. Stopping the server startup would be a breaking and inconvenient change for those users. The opposite might also happen, with users adding new options to the config files to prepare an update.