Add base_path as constant in generated gateway files

[lelvisl]

🚀 Feature

Add base_path as constant in generated gateway files will be a good feature.
I try to mount two grpc services on one http router

	schema.RegisterStorageServer(grpcServer, apiSvc)
	schema.RegisterExternalServiceServer(grpcServer, apiSvc.ExternalService)
	schema.RegisterDocumentsServiceServer(grpcServer, apiSvc.DocumentsService)
	externalGateway := runtime.NewServeMux(runtime.WithMarshalerOption(runtime.MIMEWildcard, &runtime.JSONPb{OrigName: true, EmitDefaults: true}))
	documentsGateway := runtime.NewServeMux(runtime.WithMarshalerOption(runtime.MIMEWildcard, &runtime.JSONPb{OrigName: true, EmitDefaults: true}))

	err = schema.RegisterExternalServiceHandlerServer(ctx, externalGateway, apiSvc.ExternalService)
	err = schema.RegisterDocumentsServiceHandlerServer(ctx, documentsGateway, apiSvc.DocumentsService)
	externalBasePath:="/api/external/v1"
	documentsBasePath:="/api/external/v1"

	router := chi.NewRouter()
	router.Mount(externalBasePath, http.StripPrefix(externalBasePath, externalGateway))
	router.Mount(documentsBasePath, http.StripPrefix(documentsBasePath, externalGateway))

If externalBasePath and documentsBasePath are in the generated files - it will be very useful.

[johanbrandhorst]

What would be the source of truth for this variable? The existing swagger annotation is not used by the gateway generator and we're unlikely to change that just for this feature.

[lelvisl]

yes, i thought about annotation in protofile (grpc.gateway.protoc_gen_swagger.options.openapiv2_swagger).

we're unlikely to change that just for this feature.

why?

[johanbrandhorst]

It will be harder than you think to add this, and I don't think we want to add the maintenance cost of supporting both the http annotations and swagger annotations in the gateway generator. The swagger annotations affect only the swagger output, and not the output of the other generator, and that keeps things simpler. This feature is not important enough for us to change that. Sorry.

[lelvisl]

if i implement it myself - will you accept this code?

[johanbrandhorst]

No, because it's us maintainers who have to maintain the code. I don't think this feature is important enough to;

1. Require protoc-gen-grpc-gateway to know about the swagger annotations
2. To break with the current practice of not having the swagger annotations affect the generated Go code, e.g. it is only for modifying the generated OpenAPI spec.