depends on what you want to do with etcd. I guess a filebeat module for etcd log files and a metricbeat module for metrics sounds most plausible.
Note: the beats also act as framework. That is you can build your own beat by using metricbeat as framework and have only your own module compiled in. This allows you to experiment with modules, without having to fork metricbeat.
Advantage of filebeat/metricbeat modules is: you can try to push them upstream. This way the project/modules can live and will be maintained in the official beats repositories.