Sorry, I think I misunderstood.
If you are extending metricbeat you will be either adding a new set of metrics (metricset) to an existing module, or creating a new module if the system you want to monitor doesn't already exists here:
Either case, you fork the repo, and probably the first thing to do is building the binary without any modification to make sure your environment is set.
You will find a minimal
metricbeat.yml file here https://github.com/elastic/beats/blob/master/metricbeat/metricbeat.yml. You can use for testing your generated binary.
Can you try to compile and run
metricbeat without modifying anything yet?
If I'm understanding correctly you don't want a new beat, but extend current metricbeat functionality, in such case
/mybeat shouldn't exist.