ServletAdapter.HttpServlet.doGet(HttpServletRequest,
HttpServletResponse) to serve gRPC GET request.HttpServlet.doPost(HttpServletRequest,
HttpServletResponse) to serve gRPC POST request.HttpServletRequest may come from a gRPC client.HttpServletRequest into gRPC request and lets a gRPC server
process it, and transforms the gRPC response into HttpServletResponse.UnsupportedOperationException.