clientv3: let etcd client use all available keepalive ClientParams

We should allow etcd client use all of the available keepalive
client parameters as documented in this link,
https://godoc.org/google.golang.org/grpc/keepalive#ClientParameters
Currently in the etcd, by default PermitWithoutStream is set to
false, and user has no way to override it.
On the server side, we explicitely setting EnforcementPolicy
PermitWithoutStream to false and don't provide option to override it
to user but on the client side we should allow this option as
provided by the grpc.

clientv3/client.go

@@ -233,8 +233,9 @@ func (c *Client) processCreds(scheme string) (creds *credentials.TransportCreden
 func (c *Client) dialSetupOpts(creds *credentials.TransportCredentials, dopts ...grpc.DialOption) (opts []grpc.DialOption, err error) {
 	if c.cfg.DialKeepAliveTime > 0 {
 		params := keepalive.ClientParameters{
-			Time:    c.cfg.DialKeepAliveTime,
-			Timeout: c.cfg.DialKeepAliveTimeout,
+			Time:                c.cfg.DialKeepAliveTime,
+			Timeout:             c.cfg.DialKeepAliveTimeout,
+			PermitWithoutStream: c.cfg.PermitWithoutStream,
 		}
 		opts = append(opts, grpc.WithKeepaliveParams(params))
 	}

clientv3/config.go

@@ -78,6 +78,9 @@ type Config struct {
 	// If nil, use the default logger.
 	// TODO: configure gRPC logger
 	LogConfig *zap.Config
+
+	// PermitWithoutStream, if set true, client can send keepalive pings to server without any active streams(RPCs).
+	PermitWithoutStream bool `json:"permit-without-stream"`
 }
 
 // DefaultLogConfig is the default client logging configuration.