tsnet: split user facing and backend logging

This adds a new `UserLogf` field to the `Server` struct.
When set this any logs generated by Server are logged using
`UserLogf` and all spammy backend logs are logged to `Logf`.

If it `UserLogf` is unset, we default to `log.Printf` and
if `Logf` is unset we discard all the spammy logs.

Fixes #12094

Signed-off-by: Maisem Ali <maisem@tailscale.com>

cmd/tsidp/tsidp.go

@@ -40,7 +40,6 @@ import (
 	"tailscale.com/tsnet"
 	"tailscale.com/types/key"
 	"tailscale.com/types/lazy"
-	"tailscale.com/types/logger"
 	"tailscale.com/types/views"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/must"
@@ -95,8 +94,8 @@ func main() {
 		ts := &tsnet.Server{
 			Hostname: "idp",
 		}
-		if !*flagVerbose {
-			ts.Logf = logger.Discard
+		if *flagVerbose {
+			ts.Logf = log.Printf
 		}
 		st, err = ts.Up(ctx)
 		if err != nil {