prober: add labels to Probe instances.
This allows especially dynamically-registered probes to have a bunch
more dimensions along which they can be sliced in Prometheus.
Signed-off-by: David Anderson <danderson@tailscale.com>
Turns out, it's annoying to have to wait the entire interval
before getting any monitorable data, especially for very long
interval probes like hourly/daily checks.
Signed-off-by: David Anderson <danderson@tailscale.com>
