It seems, that by design AM does not allow the end user to customize the log pattern layout.
Some 3rd Party 'log miner' products may have hard limitations (max length) on the log entry (record) being consumed, and need a way to adjust the log (like trim the first parameter so as to ONLY show the class NAME!, not full path). Please see the related case here 29784 for details on the customer's third party product.
Thus, this is really an AM infrastructure request for working with containers like tomcat (first or mostly) so customers can make any log pattern adjustments on their own via a log4j.properties file or some other control setting that governs the conversion pattern as described here.
(RFE sidenote: From a (chat) discussion it looks like this has been on the RFE backlog for some time now, but I couldn't find any 'direct hit' for an open RFE to date. But if one exists, then kindly do the needful and reject this one and link the 'old' one. And please pardon the mess (duplicate) if one already exists like OPENAM-10828 = closed in 12.0.0.