Expose RPC reconnections and ping time (#20157)

- Keeps track of reconnection count.
- Keeps track of connection ping roundtrip times. 
  Sends timestamp in ping message.
- Allow ping without payload.

internal/grid/msg.go

@@ -290,10 +290,19 @@ func (muxConnectError) Op() Op {
 }
 
 type pongMsg struct {
-	NotFound bool    `msg:"nf"`
-	Err      *string `msg:"e,allownil"`
+	NotFound bool      `msg:"nf"`
+	Err      *string   `msg:"e,allownil"`
+	T        time.Time `msg:"t"`
 }
 
 func (pongMsg) Op() Op {
 	return OpPong
 }
+
+type pingMsg struct {
+	T time.Time `msg:"t"`
+}
+
+func (pingMsg) Op() Op {
+	return OpPing
+}