input directory path, go module;package flag, gRPC ServiceConfig path flag, API service config path flag, release level
google/cloud/texttospeech/v1, --go-gapic-package cloud.google.com/go/texttospeech/apiv1;texttospeech, --grpc-service-config google/cloud/texttospeech/v1/texttospeech_grpc_service_config.json, --gapic-service-config google/cloud/texttospeech/v1/texttospeech_v1.yaml, --release-level alpha