add support for exim4

[ssbarnea]

Please add support for exim4

[celenechang]

Thanks @ssbarnea. We're interested in doing this but probably won't get to it in the near future. You're welcome to submit a PR for us to review if you'd like.

[ssbarnea]

Any change on getting this fixed or at least providing a link so something similar, so we can update the code to make it work for exim?

I suspect that this would require parsing logs in order to count messages in/out/filtered.

[JeanFred]

See also DataDog/integrations-core#1721