Support setting veneur_metrics_scopes via environment variables

[jonasschmidt]

Hey,

I did not find a way to set veneur_metrics_scopes via environment variables. After digging a bit through the https://github.com/kelseyhightower/envconfig documentation, I don't see how that would be possible with how the corresponding Veneur config is structured here: https://github.com/stripe/veneur/blob/00be4dd6629e56b8ca3bed1f7ece7cd4089607a8/config.go#L112-L118

I tried VENEUR_VENEURMETRICSSCOPES="counter:local,gauge:local,histogram:local,set:local,status:local" and VENEUR_VENEURMETRICSSCOPES_COUNTER="local" but there seems to be no effect on where internal metrics are sent.

Maybe I'm missing something?