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?
If you don't setup a name, it's going to default to the last part of the path. I'll add an explicit note.
so for example, memstats/PauseTotalNs will be reported as go_expvar.PauseTotalNs.