Add explanatory comments.

credentials/alts/internal/handshaker/service/service.go

@@ -47,6 +47,8 @@ func Dial(hsAddress string) (*grpc.ClientConn, error) {
 	if !ok {
 		// Create a new connection to the handshaker service. Note that
 		// this connection stays open until the application is closed.
+		// Disable the service config to avoid unnecessary TXT record lookups that
+		// cause timeouts with some versions of systemd-resolved.
 		var err error
 		hsConn, err = grpc.Dial(hsAddress, grpc.WithTransportCredentials(insecure.NewCredentials()), grpc.WithDisableServiceConfig())
 		if err != nil {

internal/resolver/dns/dns_resolver.go

@@ -237,6 +237,8 @@ func (d *dnsResolver) watcher() {
 }
 
 func (d *dnsResolver) lookupSRV(ctx context.Context) ([]resolver.Address, error) {
+	// Skip this particular host to avoid timeouts with some versions of
+	// systemd-resolved.
 	if !EnableSRVLookups || d.host == "metadata.google.internal." {
 		return nil, nil
 	}