Is your feature request related to a problem? Please describe.

As discussed in #739, currently log_struct for Batch cause type error when the log level is passed in str to severity argument. To align with the document and usability, it's great to have some mechanism to convert str to appropriate Enum value.

Describe the solution you'd like

Apply some logic to convert str data to Enum like what has done for log function

Describe alternatives you've considered

Change description of the document. (ref. #739)

Additional context

#739 #741