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/tcp.go

@@ -12,9 +12,12 @@ import (
 // TCP returns a Probe that healthchecks a TCP endpoint.
 //
 // The ProbeFunc reports whether it can successfully connect to addr.
-func TCP(addr string) ProbeFunc {
-	return func(ctx context.Context) error {
-		return probeTCP(ctx, addr)
+func TCP(addr string) ProbeClass {
+	return ProbeClass{
+		Probe: func(ctx context.Context) error {
+			return probeTCP(ctx, addr)
+		},
+		Class: "tcp",
 	}
 }