[management] Let embedding binaries extend the command tree (#7483)

The management binary is embedded by downstream builds that override
server construction via SetNewServer, but the cobra command tree itself
was closed: rootCmd is unexported and fully assembled in init, with no
way to attach additional subcommands. Customize hands the built root
command to a caller-supplied function before Execute, so an embedding
binary can add its own commands next to — or under — the built-in ones,
such as extra administrative helpers beneath the existing admin group.

management/cmd/root.go

@@ -54,6 +54,15 @@ func Execute() error {
 	return rootCmd.Execute()
 }
 
+// Customize hands the fully built root command to fn so an embedding binary
+// can extend or adjust the command tree — most commonly attaching its own
+// subcommands next to (or under) the built-in ones — before calling Execute.
+// The root command is constructed in this package's init, so Customize may be
+// called from the embedding binary's main at any point before Execute.
+func Customize(fn func(root *cobra.Command)) {
+	fn(rootCmd)
+}
+
 func init() {
 	mgmtCmd.Flags().IntVar(&mgmtPort, "port", 80, "server port to listen on (defaults to 443 if TLS is enabled, 80 otherwise")
 	mgmtCmd.Flags().BoolVar(&disableLegacyManagementPort, "disable-legacy-port", false, "disabling the old legacy port (33073)")

management/cmd/root_test.go

@@ -0,0 +1,42 @@
+package cmd
+
+import (
+	"testing"
+
+	"github.com/spf13/cobra"
+)
+
+// TestCustomize verifies an embedding binary can extend the command tree: a
+// top-level command attached through the hook, and a subcommand attached under
+// the built-in admin group, are both resolvable exactly as Execute would
+// resolve them.
+func TestCustomize(t *testing.T) {
+	topLevel := &cobra.Command{Use: "some-extra", RunE: func(*cobra.Command, []string) error { return nil }}
+	nested := &cobra.Command{Use: "cluster", RunE: func(*cobra.Command, []string) error { return nil }}
+
+	Customize(func(root *cobra.Command) {
+		root.AddCommand(topLevel)
+		for _, c := range root.Commands() {
+			if c.Name() == "admin" {
+				c.AddCommand(nested)
+				return
+			}
+		}
+		t.Fatal("admin command not found in the root tree")
+	})
+	t.Cleanup(func() {
+		rootCmd.RemoveCommand(topLevel)
+		for _, c := range rootCmd.Commands() {
+			if c.Name() == "admin" {
+				c.RemoveCommand(nested)
+			}
+		}
+	})
+
+	if found, _, err := rootCmd.Find([]string{"some-extra"}); err != nil || found != topLevel {
+		t.Fatalf("top-level command not resolvable: found=%v err=%v", found, err)
+	}
+	if found, _, err := rootCmd.Find([]string{"admin", "cluster"}); err != nil || found != nested {
+		t.Fatalf("nested admin subcommand not resolvable: found=%v err=%v", found, err)
+	}
+}