Add ConstLabels option

[nsurfer]

This adds ConstLabels, a label map, to surveyor.Options. The option is not exposed via the CLI, but can be used programatically like so:

s, err := surveyor.NewSurveyor(&Options{
	ConstLabels: prometheus.Labels{
		"foo": "bar",
		"baz": "qux",
	},
})

This adds constant labels to all exported metrics. For example:

nats_core_account_bytes_recv{account="AB7W",baz="qux",foo="bar"} 14004

If no ConstLabels are set, then the exported metrics will look exactly like before.

Fixes #95

[caleblloyd]

LGTM