Try the `mutate` filter...

    filter {
        mutate {
            split => [ 'source_lonlat', ',' ]
        }
    }

If that doesn't work, maybe something more hacky with csv will with...

    filter {
        csv {
            source => 'source_lonlat',
            target => 'source_lonlat',
            columns => [ '0', '1' ]
        }
    }

Regarding go/json: apparently known issue, see: https://github.com/elasticsearch/logstash-forwarder/issues/65