Monitoring multiple NATS servers doesn't get the Server ID

[rguilmont]

Description

When monitoring multiple NATS servers, the exporter never tries to get the Server ID even though the code seems to says it should . See the following :

prometheus-nats-exporter/main.go

Lines 182 to 184 in 311823a

	// This should make the http request to get the server id
	// that is returned from /varz.
	id, url, err := parseServerIDAndURL(arg)

I'd be happy to contribute and provide a Pull Request if this behaviour is wanted.