[MDS] UI polishment for MDS support of Timeline visualization

[zhongnansu]

Tasks

• [ ] Polish error message to be data_source_name is not supported. Contact your administrator to start using multiple data sources when data source feature is not enabled Link PR:

2. Place data source name before the index name in the expression. Hierarchically user selects data source first, then index name, then metric and timestamp

• [ ] Add data source helper text into the help tooltip