Logstash - how to parse rsyslog date field from logs message into @timestamp (UTC)

I have not tested it, but you could try

grok { match => { "message" => "(?<[@metadata][ts]>\d{4} \d{2}:\d{2}:\d{2})" } }
date { match => [ "[@metadata][ts]", "MMdd HH:mm:ss" ] }

See comments on guessing the year here.