etcdmain: enable grpc tracing with --debug

etcdmain/etcd.go

@@ -382,6 +382,7 @@ func setupLogging(cfg *config) {
 	capnslog.SetGlobalLogLevel(capnslog.INFO)
 	if cfg.Debug {
 		capnslog.SetGlobalLogLevel(capnslog.DEBUG)
+		grpc.EnableTracing = true
 	}
 	if cfg.LogPkgLevels != "" {
 		repoLog := capnslog.MustRepoLogger("github.com/coreos/etcd")