The "Message" field that I am trying to achieve is the following:
"Message" : "{\"log_timestamp\":\"2016-11-04T07:30:31.944Z\",\"my_message_bytes\":\"040848036966B1\",\"log_type\":\"my_json_log\",\"source_topicArn\":\"arn:aws:sns:us-east-1:000000000000:my-feeder-devel\",\"my_site\":\"my_devel\",\"logstash_output\":\"2018-05-02T14:37:42.772Z\"}\r\n"
@Badger I have updated the input and output in my original post to reflect the actual input and actual output, respectively.
@magnusbaeck sorry for just saying "that didnt work". What I meant was that I got the identical output that I had originally gotten.