validate object implements the corresponding interface (#3724)

Signed-off-by: zouyee <zounengren@cmss.chinamobile.com>

core/dnsserver/server_https.go

@@ -14,6 +14,8 @@ import (
 	"github.com/coredns/coredns/plugin/pkg/response"
 	"github.com/coredns/coredns/plugin/pkg/reuseport"
 	"github.com/coredns/coredns/plugin/pkg/transport"
+
+	"github.com/caddyserver/caddy"
 )
 
 // ServerHTTPS represents an instance of a DNS-over-HTTPS server.
@@ -44,6 +46,9 @@ func NewServerHTTPS(addr string, group []*Config) (*ServerHTTPS, error) {
 	return sh, nil
 }
 
+// Compile-time check to ensure Server implements the caddy.GracefulServer interface
+var _ caddy.GracefulServer = &Server{}
+
 // Serve implements caddy.TCPServer interface.
 func (s *ServerHTTPS) Serve(l net.Listener) error {
 	s.m.Lock()