Is your feature request related to a problem? Please describe.
When using com.google.cloud.logging.logback.LoggingAppender, there's currently no way to flush logs explicitly (by calling getLogging().flush()), for example at the end of an HTTP request.

Describe the solution you'd like
Either increase visibility of the getLogging() method to protected, so we can extend LoggingAppender to add a flush() method, or add it directly to LoggingAppender.

Describe alternatives you've considered
A workaround is to set the flushLevel to the lowest, so every log entry is flushed immediately, but this is very inefficient.

Additional context
Here is how I would use this is a ServletFilter (Kotlin code):

	override fun doFilter(request: ServletRequest, response: ServletResponse, chain: FilterChain) {
        try {
            chain.doFilter(request, response)
        } finally {
            (LoggerFactory.getILoggerFactory() as LoggerContext).loggerList
                    .map { it.iteratorForAppenders() }
                    .forEach {
                        while (it.hasNext()) {
                            val appender = it.next()
                            when (appender.name) {
                                "CLOUD" -> (appender as LoggingAppender).flush()
                            }
                        }
                    }
        }
    }