feat(resolver/discovery): expose create watcher timeout error (#3392)

transport/grpc/resolver/discovery/builder.go

@@ -14,6 +14,8 @@ import (
 
 const name = "discovery"
 
+var ErrWatcherCreateTimeout = errors.New("discovery create watcher overtime")
+
 // Option is builder option.
 type Option func(o *builder)
 
@@ -96,7 +98,7 @@ func (b *builder) Build(target resolver.Target, cc resolver.ClientConn, _ resolv
 	case <-done:
 		err = watchRes.err
 	case <-time.After(b.timeout):
-		err = errors.New("discovery create watcher overtime")
+		err = ErrWatcherCreateTimeout
 	}
 	if err != nil {
 		cancel()