prober: export probe class and metrics from bandwidth prober

- Wrap each prober function into a probe class that allows associating
  metric labels and custom metrics with a given probe;
- Make sure all existing probe classes set a `class` metric label;
- Move bandwidth probe size from being a metric label to a separate
  gauge metric; this will make it possible to use it to calculate
  average used bandwidth using a PromQL query;
- Also export transfer time for the bandwidth prober (more accurate than
  the total probe time, since it excludes connection establishment
  time).

Updates tailscale/corp#17912

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

prober/dns_test.go

@@ -57,9 +57,9 @@ func TestForEachAddr(t *testing.T) {
 		registered = append(registered, addr)
 
 		// Return a probe that does nothing; we don't care about what this does.
-		probe := p.Run(fmt.Sprintf("website/%s", addr), probeInterval, nil, func(_ context.Context) error {
+		probe := p.Run(fmt.Sprintf("website/%s", addr), probeInterval, nil, FuncProbe(func(_ context.Context) error {
 			return nil
-		})
+		}))
 		return []*Probe{probe}
 	}