core/dnsserver: support explicit registration for embedded hosts (#8525)

* core/dnsserver: test host-plugin embedding with forward

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

* core/dnsserver: add a directive setter for embedded hosts

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

* core/dnsserver: support explicit server registration for embedded hosts

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

---------

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

core/dnsserver/example_test.go

@@ -12,6 +12,27 @@ import (
 	"github.com/miekg/dns"
 )
 
+func ExampleSetDirectives() {
+	oldDirectives := dnsserver.Directives
+	defer func() { dnsserver.Directives = oldDirectives }()
+
+	if err := dnsserver.SetDirectives([]string{"bind", "whoami"}); err != nil {
+		panic(err)
+	}
+	fmt.Println(dnsserver.Directives)
+	// Output: [bind whoami]
+}
+
+func ExampleRegister() {
+	// Required before caddy.Start in builds with coredns_manual_registration;
+	// harmless when the default import-time registration has already run.
+	if err := dnsserver.Register(); err != nil {
+		panic(err)
+	}
+	fmt.Println("registered")
+	// Output: registered
+}
+
 func Example_embedding() {
 	oldDirectives := dnsserver.Directives
 	oldCaddyQuiet := caddy.Quiet
@@ -25,6 +46,9 @@ func Example_embedding() {
 	// Import only the plugins the host needs and set their execution order
 	// before starting the first server.
 	dnsserver.Directives = []string{"bind", "whoami"}
+	if err := dnsserver.Register(); err != nil {
+		panic(err)
+	}
 	caddy.Quiet = true
 	dnsserver.Quiet = true