Rudy/golang expvars

Agent check to report values exposed in go app by expvars.

Expvar will by default expose information about the go runtime and also allow to add custom variables.


@remh Do you have an opinion on the way used to specify what metrics to report?
A python example:
https://github.com/DataDog/dd-agent/blob/3.9.3/checks/jmx_connector.py#L45-L53