Add an event dispatcher to `OC\Log`

[summersab]

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Is your feature request related to a problem? Please describe.
I have written a good deal of custom security and privacy code/apps for my Nextcloud installation. It is possible that an unknown bug in my code could result in Nextcloud writing sensitive data from my apps to the log.

Additionally, being able to filter logs based on apps, event types, severity level, etc. prior to them being written by the logger would be useful.

Describe the solution you'd like
It would be helpful if the OC\Log class included an event dispatcher. This way, logged messages could be checked by middleware before being written to the log file.

Describe alternatives you've considered
I'm sure the alternative suggestion would be, "Write better code that doesn't result in secrets being exposed," and that adding event handlers to the logger creates unnecessary complexity. I am pretty confident in the quality of my code . . . but you never know. There are always bugs in code, and having an extra mechanism in place just in case something goes wrong would be a good thing and would help prevent sensitive strings from being logged.

Also, having the ability to filter logs based on the app or event that triggered the message could be very useful for debugging and management.

[summersab]

I managed to find a way to do this by extending OC\Log and writing an app that registers my custom class to the server:
https://github.com/nunimbus/custom_logger

It would still be ideal to have an event dispatcher because my class will only catch errors AFTER the app has loaded - any errors that occur prior to this will not be filtered. For now, this will work for my purposes.

[joshtrichards]

Added in #38490