telemetry gnmi does not support multiple redis instance database get and subscription

[FengPan-Frank]

Current telemetry gnmi does not support multiple redis instance database get and subscription

Like below redis database instance:

We can get data from database via sonic-db-cli as below:

but failed with gnmi subscription

[FengPan-Frank]

#361, I've tried to make gnmi fix but blocked by functionality test, seems the issue is due to fileDescriptor2 also needs to be updated in sonic.pb.go, but it doesn't work even after I recompile the sonic.proto via "protoc --go_out=. --go_opt=paths=source_relative sonic.proto"