/topic/jsiwek/snmp in bro, binpac, and bro-testing-private adds support for parsing SNMP datagrams. It's only absent a snmp.log.
Seth, do you mind taking a look at what might make sense for a default snmp.log? I'm guessing it might look similar in concept to dns.log. A difference is I'm not sure how meaningful raw OID to value mappings will be.
The code is in a merge-able state as it is in the branch/repos I mentioned, and IMO, has value even without a default snmp.log. So if you just want to flip to a merge request and postpone thinking up an snmp.log for later, I think that's fine, too.