There is no extension to monitor swarm. You can still install machine agent in the docker host machine or as a container. I haven't tried this though.
Also, please refer to this document for 1st class support for docker with Server Visibility feature.
https://docs.appdynamics.com/display/PRO43/Integrated+Docker+Visibility https://docs.appdynamics.com/display/PRO43/Configuring+Docker+Visibility
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