util/eventbus: add debugger methods to list pub/sub types

This lets debug tools list the types that clients are wielding, so
that they can build a dataflow graph and other debugging views.

Updates #15160

Signed-off-by: David Anderson <dave@tailscale.com>

util/eventbus/debug.go

@@ -5,6 +5,7 @@ package eventbus
 
 import (
 	"fmt"
+	"reflect"
 	"slices"
 	"sync"
 	"sync/atomic"
@@ -108,6 +109,27 @@ func (d *Debugger) WatchSubscribe(client *Client) *Subscriber[DeliveredEvent] {
 	return newMonitor(client.subscribeState().debug.add)
 }
 
+// PublishTypes returns the list of types being published by client.
+//
+// The returned types are those for which the client has obtained a
+// [Publisher]. The client may not have ever sent the type in
+// question.
+func (d *Debugger) PublishTypes(client *Client) []reflect.Type {
+	d.checkClient(client)
+	return client.publishTypes()
+}
+
+// SubscribeTypes returns the list of types being subscribed to by
+// client.
+//
+// The returned types are those for which the client has obtained a
+// [Subscriber]. The client may not have ever received the type in
+// question, and here may not be any publishers of the type.
+func (d *Debugger) SubscribeTypes(client *Client) []reflect.Type {
+	d.checkClient(client)
+	return client.subscribeTypes()
+}
+
 // A hook collects hook functions that can be run as a group.
 type hook[T any] struct {
 	sync.Mutex