protected final void log(Level level, Object... msgParts) {
        if (!log.isLoggable(level)) return;

        StringBuilder s = new StringBuilder();
        for (Object part : msgParts)
            s.append(part.toString());
        log.log(level, s.toString());
    }
Returns the next message in the stream, or null if none
vespaclient-container-plugin/src/main/java/com/yahoo/vespa/http/server/ClientFeederV3.java