add mongodb OP_MSG (2013)

[pohzipohzi]

this fixes #6191

this is a continuation of #8594 after running into problems with the CLA. It was created by saving changes onto a patch and applying them onto a new branch opc2

git diff origin/master... --binary > ~/Desktop/useful/beats.patch
git checkout master
git branch -b opc2
git apply ~/Desktop/useful/beats.patch
(commit and make PR)

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[pohzipohzi]

@adriansr could you help to review this?

[adriansr]

Thanks for taking the time to finish this feature.

I found a small problem, everything else is correct.

Also, I took the liberty to edit some formatting changes to beat.py that were probably added by your editor.