There’s an open issue (a support request) discussion on Github that seems somewhat related to what you're trying to achieve. I assume the Ops Agent does not yet support custom multiline parsing. According to the discussion, someone mentioned that an internal tracking bug has already been filed for this request.

I would also like to suggest that you file a feature request on Google’s public issue tracker, so their product engineering team is aware that more users are interested in this type of use case.However, please keep in mind that there is no ETA for a response or any assurance that it will be implemented promptly.