ipn/localapi, cli, clientmetric: add ipnbus feature tag; fix omit.go stub

Add a new "ipnbus" build feature tag so the watch-ipn-bus LocalAPI
endpoint can be independently controlled, rather than being gated
behind HasDebug || HasServe. Minimal/embedded builds that omit both
debug and serve were getting 404s on watch-ipn-bus, breaking
"tailscale up --authkey=..." and other CLI flows that depend on
WatchIPNBus.

In the CLI, check buildfeatures.HasIPNBus before attempting to watch
the IPN bus in "tailscale up"/"tailscale login", and exit early with
an informational message when the feature is omitted.

Also add the missing NewCounterFunc stub to clientmetric/omit.go,
which caused compilation errors when building with
ts_omit_clientmetrics and netstack enabled.

Fixes #19240

Change-Id: I2e3c69a72fc50fa02542b91b8a54859618a463d1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/up.go

@@ -599,6 +599,11 @@ func runUp(ctx context.Context, cmd string, args []string, upArgs upArgsT) (retE
 		}
 	}()
 
+	if !buildfeatures.HasIPNBus {
+		fmt.Fprintln(Stderr, "binary built with ts_omit_ipnbus; not waiting for completion")
+		return nil
+	}
+
 	// Start watching the IPN bus before we call Start() or StartLoginInteractive(),
 	// or we could miss IPN notifications.
 	//