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/http.go

@@ -13,14 +13,17 @@ import (
 
 const maxHTTPBody = 4 << 20 // MiB
 
-// HTTP returns a Probe that healthchecks an HTTP URL.
+// HTTP returns a ProbeClass that healthchecks an HTTP URL.
 //
-// The ProbeFunc sends a GET request for url, expects an HTTP 200
+// The probe function sends a GET request for url, expects an HTTP 200
 // response, and verifies that want is present in the response
 // body.
-func HTTP(url, wantText string) ProbeFunc {
-	return func(ctx context.Context) error {
-		return probeHTTP(ctx, url, []byte(wantText))
+func HTTP(url, wantText string) ProbeClass {
+	return ProbeClass{
+		Probe: func(ctx context.Context) error {
+			return probeHTTP(ctx, url, []byte(wantText))
+		},
+		Class: "http",
 	}
 }