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:

veneur/config.go

Lines 112 to 118 in 00be4dd

	VeneurMetricsScopes struct {
	Counter string `yaml:"counter"`
	Gauge string `yaml:"gauge"`
	Histogram string `yaml:"histogram"`
	Set string `yaml:"set"`
	Status string `yaml:"status"`
	} `yaml:"veneur_metrics_scopes"`

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?