mimir.proto is copied from Grafana's mimir repo. It is using gogoproto for generating go code. However, if you don't like to use gogoproto. Simply comment all gogo lines, it should work. Steps are:
Install grpc library, tool, and snappy for python
pip install python-snappy python3 -m pip install grpcio python3 -m pip install grpc-tools
Generate python code using following cmd (assume you run it in the dir where mimir.proto is located): />python3 -m grpc_tools.protoc -I./ --python_out=. --pyi_out=. --grpc_python_out=. ./mimir_python.proto