Allow LoggerBackend to take a function to determine whether or not forward a message to Sentry

[aselder]

We use Sentry extensively and the LoggerBackend to send message to Sentry. Often, there are 3rd party libraries that logging things with Logger.error that we don't care about.

I'd love to be able to pass a function to the LoggerBackend config which would take the log message and return true or false on whether to forward that message to Sentry, very similar the the filter option available to the top level sentry config.

I could throw together a PR if people think this would be useful and valuable.