Provide a link or URL to `/metrics` on the `/` request

[grahamc]

curl 127.0.0.1:9465 gets me a 404. It seems that most metrics exporter in the ecosystem returns a notice that the metrics are at /metrics. Since I've seen this before I thought to look there, but it would be nice for prometheus-packet-sd to also return that notice.

[t0mk]

@grahamc can you please link to some Prometheus SD Provider repo which indeed exposes the /metrics on /?
I ask for both confirmation, and for code example :D