I am going through the docs about logging and I am not all clear on what i can do with the webhooks for logs .
But usually I used logstash to filter mail log data from file and push it into elasticseach.
But as far as I have researched logstash cannot read zstd logfiles and neither is rsyslog able to do that.
So that leaves me with the only option of using a http input service in logstash and push logs from KumoMTA using a webhook into logstash?
Is my thinking right? Any other ideas?
Well I will answer my question partly myself 
I did use the log_hooks:new_json way to create a webhook to a http-listener on logstash. And for now I have the json formatted info in logstash. From there I can modify the data in logstash to fti my needs and merge it in an existing format that I already have.
Not done yet with the project, but it is looking pretty good. Thanks for the good documentation. It helped my out.
Perfect. That is exactly what I would have suggested. Webhooks is a good solution for most remote logging systems.