https://aip.dev/client-libraries/4222

It looks like the generator is not adding variables declared in the google.api.http annotation to routing headers.

https://github.com/googleapis/googleapis/blob/52701da10fec2a5f9796e8d12518c0fe574488fe/google/cloud/servicedirectory/v1beta1/registration_service.proto#L60-L65

See https://github.com/GoogleCloudPlatform/python-docs-samples/pull/3456/files#r412305714 for the modifications that need to be made to currently use the library.

Possibly related to #380.

Metadata should be added somewhere here (see this method for comparison))