add timing info during ping operation

weed/pb/master.proto

@@ -343,6 +343,9 @@ message PingRequest {
   string target_type = 2;
 }
 message PingResponse {
+  int64 start_time_ns = 1;
+  int64 remote_time_ns = 2;
+  int64 stop_time_ns = 3;
 }
 
 message RaftAddServerRequest {
@@ -369,4 +372,4 @@ message RaftListClusterServersResponse {
     string suffrage = 3; //
   }
   repeated ClusterServers cluster_servers = 1;
-}
+}