Do not start / expose metrics by default

[binarylogic]

Related #230

We should not expose a Prometheus specific metrics endpoint by default. I'm assuming #230 is a better solution to this, until then I think this should be explicitly enabled.

[LucioFranco]

I'd like to add to this, that this should also not set the metrics subscriber as it is a bit heavy right now (internally uses locks) which will go away soon but if I disable metrics I expect for those costs to not exist. So this means that instead of wrapping the tokio-trace-fmt subscriber we should just set that as the sub.

This is the line where it wraps the FmtSubscriber https://github.com/timberio/vector/blob/master/src/main.rs#L58

[michaelfairley]

@LucioFranco
tokio_trace::subscriber::with_default, can't take a dyn Subscriber, so I can't pass it a Boxed subscriber, so there's no simple way for me to sometimes have a MetricsSubscriber and sometimes not. I think my 2 best options are:

1. Always use the MetricsSubscriber. #YOLO
2. Write an EitherSubscriber that implements Subscriber and forwards all of its calls to one of the two concrete implementations.

Any thoughts?