Variables to capture GRPC response- status code

[padoleaish]

As per our requirement, we wanted to capture GRPC response status code for logging purpose. 

We tried to search below link for variables, but couldn't find variable for GRPC response code.

https://cloud.google.com/apigee/docs/api-platform/reference/variables-reference

Can you please tell us, how to get the GRPC status code in response.

@dchiesa1 @dknezic @ganadurai @shaaland @solomonjason